Fix ${!foo@} and ${!foo*} to include 'foo' itself in search

These expansions are supposed to yield all variable names beginning
with the indicated prefix. This should include the variable name
that is identical to the prefix (as 'prefix' begins with 'prefix').

This bugfix is backported from the abandoned ksh 93v- beta, so AT&T
intended this change. It also makes ksh work like bash in this.

src/cmd/ksh93/sh/macro.c: varsub(): M_NAMESCAN:
- Check if the prefix itself exists. If so, start with that.

src/cmd/ksh93/tests/variables.sh:
- Add tests for these expansions.

src/cmd/ksh93/sh.1:
- Fix the incomplete documentation of these expansions.

src/cmd/ksh93/COMPATIBILITY:
- Note the change as it's potentially incompatible in corner cases.

Resolves: https://github.com/ksh93/ksh/issues/183

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-03-09:
+
+- The ${!foo@} and ${!foo*} expansions yield variable names beginning with foo,
+  but excluded 'foo' itself. The fix for this is now backported from 93v- beta.
+
 2021-03-07:
 
 - Fixed the typeset -p display of short integers without an assigned value.