Add support for saving exit status into file

Since exit status of code-executing assert functions cannot be
guarranteed to match exit status of the code being executed, there is
now a way to get the latter in more reliable way.  This should be also
useful in cases when there are further asserts on value of the status.

src/jat.sh.skel

     #
     # Usage:
     #
-    #     jat__cmd [-o R_OUT] [-e R_ERR] [-S ES_EXPR] [--] CMD [ARG]..
+    #     jat__cmd [-o R_OUT] [-e R_ERR] [-s R_ESF] [-S ES_EXPR] [--] CMD [ARG]..
     #
     # Run CMD with all ARGs and if exit status is zero, announce assertion
     # success (PASS), otherwise announce assertion failure (FAIL).
     # thereof.  E, g. expression '0,10-20' would be a valid expression,
     # matching 0, 11, 20, but not 2 or 21.
     #
+    # If R_ESF is passed, it will be treated as path to file where exit status
+    # of CODE is written.  This is recommended over consulting $?, since the
+    # latter is not reliable; see below.
+    #
     # If path R_OUT or R_ERR are provided, standard output and standard
     # error are saved to respective files.
     #
     # See COMMON ARGUMENTS section for common assert options.
     #
+    # Exit status of the this function normally will, but IS NOT GUARRANTEED TO
+    # correspond to the exit status of the CODE. For example, if the function
+    # call is incomplete, the exit status will be 2.
+    #
     local __jat__cmd=()
     local __jat__r_es
     local __jat__o_es=0
     local __jat__etype=TEST_ERROR
     local __jat__r_out
     local __jat__r_err
+    local __jat__r_esf
     #
     # NOTE: names need to be qualified because they might interfere
     #       with the actual execution of the assert command.
         -h) __jat__hint=$2; shift 2 || { __jat__usage "missing HINT"; return 2; } ;;
         -e) __jat__r_err=$2; shift 2 || { __jat__usage "missing R_ERR"; return 2; } ;;
         -o) __jat__r_out=$2; shift 2 || { __jat__usage "missing R_OUT"; return 2; } ;;
+        -s) __jat__r_esf=$2; shift 2 || { __jat__usage "missing R_ESF"; return 2; } ;;
         -S) __jat__o_es=$2; shift 2 || { __jat__usage "missing ES_EXPR"; return 2; } ;;
         --) shift; break ;;
         *) break ;;
         *:0) "${__jat__cmd[@]}" >"$__jat__r_out" ;;
         *:*) "${__jat__cmd[@]}" >"$__jat__r_out" 2>"$__jat__r_err" ;;
     esac; __jat__r_es=$?
-    debug -v __jat__r_es
+    debug -v __jat__r_es __jat__r_esf
+    if test -n "$__jat__r_esf"; then
+        echo $__jat__r_es > "$__jat__r_esf" \
+         || jat__log_error "error writing R_ESF file: $__jat__r_esf"
+    fi
     if __jat__es_match "$__jat__o_es" "$__jat__r_es"; then
         __jat__etype=PASS
     else
     #
     # Usage:
     #
-    #     jat__eval [-S ES_EXPR] [--] CODE
+    #     jat__eval [-s R_ESF] [-S ES_EXPR] [--] CODE
     #
     # Run CODE using eval builtin and if exit status is zero, announce assertion
     # success (PASS), otherwise announce assertion failure (FAIL).
     # thereof.  E, g. expression '0,10-20' would be a valid expression,
     # matching 0, 11, 20, but not 2 or 21.
     #
-    # This is similar to jat__cmd(), except that code is checked agains syntax
-    # errors and user is given slightly full control over redirections.
+    # If R_ESF is passed, it will be treated as path to file where exit status
+    # of CODE is written.  This is recommended over consulting $?, since the
+    # latter is not reliable; see below.
+    #
+    # This is similar to jat__cmd(), except that code is checked against syntax
+    # errors and user is given full control over redirections.
     #
     # See COMMON ARGUMENTS section for common assert options.
     #
+    # Exit status of the this function normally will, but IS NOT GUARRANTEED TO
+    # correspond to the exit status of the CODE. For example, if the function
+    # call is incomplete, the exit status will be 2.  If there is syntax error
+    # in CODE, the exit status will be 3.
+    #
     local __jat__code=""
     local __jat__r_es
+    local __jat__r_esf
     local __jat__o_es=0
     local __jat__hint
     local __jat__beids=()
     while true; do case $1 in
         -b) __jat__beids+=("$2"); shift 2 || { __jat__usage "missing BEID"; return 2; } ;;
         -h) __jat__hint=$2; shift 2 || { __jat__usage "missing HINT"; return 2; } ;;
+        -s) __jat__r_esf=$2; shift 2 || { __jat__usage "missing R_ESF"; return 2; } ;;
         -S) __jat__o_es=$2; shift 2 || { __jat__usage "missing ES_EXPR"; return 2; } ;;
         --) shift; break ;;
         *) break ;;
     }
     jat__log_info "CODE: $__jat__code"
     eval "$__jat__code"; __jat__r_es=$?
+    if test -n "$__jat__r_esf"; then
+        echo $__jat__r_es > "$__jat__r_esf" \
+         || jat__log_error "error writing R_ESF file: $__jat__r_esf"
+    fi
     debug -v __jat__r_es
     if __jat__es_match "$__jat__o_es" "$__jat__r_es"; then
         __jat__etype=PASS