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

src/cmd/ksh93/data/testops.c

@@ -57,7 +57,7 @@ const Shtable_t shtab_testops[] =
 };
 
 const char sh_opttest[] =
-"[-1c?\n@(#)$Id: test (ksh 93u+m) 2020-08-31 $\n]"
+"[-1c?\n@(#)$Id: test (ksh 93u+m) 2021-11-13 $\n]"
 "[--catalog?" SH_DICT "]"
 "[+NAME?test, [ - evaluate expression]"
 "[+DESCRIPTION?\btest\b evaluates expressions and returns its result using the "
@@ -97,7 +97,7 @@ const char sh_opttest[] =
 	"[+?Operators marked with a * are not part of the POSIX standard.]"
 "}"
 "[+UNARY OPERATORS?These evaluate as True if:]{"
-	"[+-a \afile\a *?Same as \b-e\b.]"
+	"[+-a \afile\a *?Deprecated. Same as \b-e\b.]"
 	"[+-b \afile\a?\afile\a is a block special file.]"
 	"[+-c \afile\a?\afile\a is a character special file.]"
 	"[+-d \afile\a?\afile\a is a directory.]"