Similar to r251236, r251242, and r251244. Introduce new function-pair to
dialog(1) API in dialog.subr responsible for retrieving menu-choice data
(for the --menu widget).

Add f_dialog_menuitem_store()/f_dialog_menuitem_fetch() for storing and
retrieving the menuitem (not the tag) from a --menu widget result.

The dialog --menu widget returns the `tag' from the tag/item pair of choices
for a menu list. Because the menu list is often local to the function that
is displaying it, the ability to dereference this into the item is not
possible unless you use a global. This changes things so the function (upon
successful return of dialog) dereferences the tag into the item and stores
it for later retrieval using these functions.

NOTE: Tags are dereferenced into items using f_dialog_menutag2item().
Comments.
Use a case-statement instead of multiple (separate even) if-statements.
Use f_shell_escape() instead of forking to awk. In this case, the
replacement comes with a great performance increase (as f_shell_escape()
uses the built-in based f_replaceall() which out-performs forking to
awk(1)). This should also improve readability slightly.