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

src/cmd/ksh93/sh.1

@@ -1274,9 +1274,14 @@ and
 inclusive using the same quoting rules as
 .BR @ .
 .TP
+.PD 0
+\f3${!\fP\f2prefix\^\fP\f3@}\fP
+.TP
 \f3${!\fP\f2prefix\^\fP\f3*}\fP
-Expands to the names of the variables whose names begin with
+These both expand to the names of the variables whose names begin with
 .IR prefix .
+The expansions otherwise work like \f3$@\fP and \f3$*\fP, respectively
+(see under \f2Quoting\fP below).
 .TP
 \f3${\fP\f2parameter\^\fP\f3:\-\fP\f2word\^\fP\f3}\fP
 If