Fix handling of '--posix' and '--default' (#265)

src/cmd/ksh93/sh/args.c: sh_argopts():
- Remove special-casing for --posix (see also data/builtins.c) and
  move the case -5: to the case ':' instead, so this option is
  handled like all other long options. This change fixes two bugs:
  1. 'set --posix' had no effect on the letoctal or braceexpand
     options. Reproducer:
       $ set --posix
       $ [[ -o braceexpand ]]; echo $?
       0
       $ [[ -o letoctal ]]; echo $?
       1
  2. 'ksh --posix' could not run scripts correctly because it
     wrongly enabled '-c'. Reproducer:
       $ ksh --posix < <(echo 'exit 0')
       ksh: -c requires argument
       Usage: ksh [--posix] [arg ...]
       Help: ksh [ --help | --man ] 2>&1
- Don't allow 'set --default' to unset the restricted option.

src/cmd/ksh93/tests/options.sh:
- Add regression tests for the bugs described above, using -o posix
  and --posix.

src/cmd/ksh93/tests/restricted.sh:
- Add a regression test for 'set --default' in rksh.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/cmd/ksh93/include/version.h

@@ -20,7 +20,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-04-08"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-04-09"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */