Add -o|--only option to list modules from only one directory

Useful when working with a private set of modules; to see what's inside,
etc.

src/bin/sfdoc

             "-I PTH, --include PTH  Include path PTH when searching"    \
             "                   for modules; can be specified multiple" \
             "                   times."                                 \
+            "-o PTH, --only-from PTH  Searcho only under path PTH."     \
         --                                                              \
         "Without command specified, will try to display documentation"  \
         "in man(1) pager."
         -I|--include)   SHELLFU_PATH="$2:$SHELLFU_PATH"; shift 2 || usage ;;
         -l|--ls)        action=lsx;                 shift; break ;;
         -L|--lsmod)     action=lsm;                 shift; break ;;
+        -o|--only-from) SHELLFU_PATH="$2"; SHELLFU_INCLUDE=""; shift 2 || usage ;;
         --lsvar)        action=lsv;                 shift; break ;;
         --lsfun)        action=lsf;                 shift; break ;;
         -e|--export)    action=exp; format="$2";    shift 2 || usage; break ;;