test/[: Fix binary -a/-o operators in POSIX mode

POSIX requires
	test "$a" -a "$b"
to return true if both $a and $b are non-empty, and
	test "$a" -o "$b"
to return true if either $a or $b is non-empty.

In ksh, this fails if "$a" is '!' or '(' as this causes ksh to
interpret the -a and -o as unary operators (-a being a file
existence test like -e, and -o being a shell option test).

$ test ! -a ""; echo "$?"
0		(expected: 1/false)
$ set -o trackall; test ! -o trackall; echo "$?"
1		(expected: 0/true)
$ test \( -a \); echo "$?"
ksh: test: argument expected
2		(expected: 0/true)
$ test \( -o \)
ksh: test: argument expected
2		(expected: 0/true)

Unfortunately this problem cannot be fixed without risking breakage
in legacy scripts. For instance, a script may well use
	test ! -a filename
to check that a filename is nonexistent. POSIX specifies that this
always return true as it is a test for the non-emptiness of both
strings '!' and 'filename'.

So this commit fixes it for POSIX mode only.

src/cmd/ksh93/bltins/test.c: e3():
- If the posix option is active, specially handle the case of
  having at least three arguments with the second being -a or -o,
  overriding their handling as unary operators.

src/cmd/ksh93/data/testops.c:
- Update 'test --man --' date and say that unary -a is deprecated.

src/cmd/ksh93/sh.1:
- Document the fix under the -o posix option.
- For test/[, explain that binary -a/-o are deprecated.

src/cmd/ksh93/tests/bracket.sh:
- Add tests based on reproducers in bug report.

Resolves: https://github.com/ksh93/ksh/issues/330

NEWS

@@ -3,6 +3,14 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-11-13:
+
+- The test/[ built-in command is fixed so that the binary -a (and) and -o (or)
+  operators, as in [ "$a" -a "$b" ] or [ "$a" -o "$b" ], work even if "$a" is
+  '!' or '('. To avoid breaking backwards compatibility with the nonstandard
+  unary [ -a "$file" ] and [ -o "$option" ] operators in combination with '!'
+  or parentheses, this fix is only activated if the posix option is on.
+
 2021-11-07:
 
 - Fixed a bug that could corrupt output if standard output is closed upon