Fix set +r so that it cannot unset the restricted option

The ksh man page documents that the restricted option cannot be
unset once it is set, which means `set +r` should be invalid.
While this was true for `set +o restricted`, `set +r` was causing
the restricted option to be unset. The fix for this problem comes
from one of Solaris' patches, which adds an error check to prevent
this behavior.

Solaris' patch:
https://github.com/oracle/solaris-userland/blob/master/components/ksh93/patches/020-CR6919590.patch

src/cmd/ksh93/sh/args.c:
 - Add an error check to stop `set +r` from unsetting the
   restricted option.

src/cmd/ksh93/tests/restricted.sh:
 - Add two regression tests to make sure the restricted option
   cannot be unset.

(cherry picked from commit bef4fee404d8e24b38fce66420c14a39ac4a123e)

src/cmd/ksh93/sh/args.c

@@ -302,6 +302,8 @@ int sh_argopts(int argc,register char *argv[], void *context)
 		}
 		else
 		{
+			if ((o == SH_RESTRICTED) && sh_isoption(SH_RESTRICTED))
+				errormsg(SH_DICT,ERROR_exit(1),e_restricted,"r"); /* set -r cannot be unset */
 			if(o==SH_XTRACE)
 				trace = 0;
 			off_option(&newflags,o);

src/cmd/ksh93/tests/restricted.sh

@@ -87,4 +87,10 @@ for i in PATH ENV FPATH
 do	check_restricted  "function foo { typeset $i=foobar;};foo" || err_exit "$i can be changed in function by using typeset"
 done
 
+# ======
+# `set +r` and `set +o restricted` should not unset the restricted option
+check_restricted 'set +r' 2> /dev/null || err_exit '`set +r` unsets the restricted option'
+check_restricted 'set +o restricted' 2> /dev/null || err_exit '`set +o restricted` unsets the restricted option'
+
+# ======
 exit $((Errors<125?Errors:125))