Check mkit.ini version

This is the last version where we tolerate incompatible changes in
mkit.ini and from now on, all mkit.ini files must have the version line
(in first 3 or last 3 lines).

As per my interpretation of SemVer, if major version is 0, we may
introduce incompatible changes within minor, in that case we check that
as well.  Otherwise, we just check the major.

src/include/mkit.sh

     exit 4
 }
 
+_compver() {
+    #
+    # True if version $1 matches our version
+    #
+    # If our x is 0, check first two fragments, otherwise check just
+    # the x.  Fragments must equal.
+    #
+    local their_ver our_x our_y their_x their_y
+    their_ver="$1"
+    their_x=${their_ver%%.*}
+    their_y=${their_ver##$their_x.}
+    their_y=${their_y%%.*}
+    our_x=${MKIT_VERSION%%.*}
+    our_y=${MKIT_VERSION##$our_x.}
+    our_y=${our_y%%.*}
+    debug_var MKIT_VERSION our_x our_y their_ver their_x their_y
+    test "$their_x" -eq "$our_x" || return 1
+    test "$our_x" -eq 0 && {
+        test "$their_y" = "$our_y"
+        return $?
+    }
+    return 0
+}
+
+_chkiniversion() {
+    #
+    # Check if ini version is supported
+    #
+    # Look for "#mkit version=0.0.0" or similar in first or last
+    # 3 lines of the file; then check if the version is supported.
+    #
+    local ver_line
+    local their_ver
+    ver_line=$(
+        {
+            head -3 "$MKIT_INI"
+            tac "$MKIT_INI" | tail -3
+        } | grep -m 1 -E '^# *mkit +version *= *v?[0-9]+\.[0-9]+\.[0-9]+'
+    )
+    test -n "$ver_line" \
+     || die "version mark ('#mkit version=x.y.z') not found in: $MKIT_INI"
+    their_ver="$(tr -d '[:blank:]v' <<<"${ver_line##*=}")"
+    _compver "$their_ver" \
+     || die "bad mkit.ini version: $their_ver does not match $MKIT_VERSION"
+}
+
 mkit_init() {
     #
     # Do basic initialization
     $MKIT_DRY && MKIT_DEBUG=true
     MKIT_PROJ_PKGNAME=$(ini 1value "project:pkgname")
     test -f "$MKIT_INI" || die "cannot find mkit.ini: $MKIT_INI"
+    _chkiniversion
     test -n "$(tr -d '[:space:]' <<<"$MKIT_LOCAL")" \
      || die "MKIT_LOCAL must be non-blank: '$MKIT_LOCAL'"
 }