set --default: properly restore ksh IFS behaviour (re: 9e2a8c69)

Reproducer:

$ (IFS=$'\t\t'; val=$'\tone\t\ttwo\t'; set --posix; \
   set -- $val; echo $#; set --noposix; set -- $val; echo $#)
2
4   <== OK

$ (IFS=$'\t\t'; val=$'\tone\t\ttwo\t'; set --posix; \
   set -- $val; echo $#; set --default; set -- $val; echo $#)
2
2   <== bug

The output of the seconnd command line should be like the first.

When POSIX mode is turned off using 'set --noposix' (or 'set +o
posix'), sh.ifstable is invalidated as it needs to be repopulated
on the next field split to restore ksh-specific special handling of
a repeated $IFS whitespace character as non-whitespace. However,
when 'set --default' is used, this does not happen, which is a bug.

src/cmd/ksh93/sh/args.c: sh_argopts():
- While processing --default, when turning off SH_POSIX, call
  sh_invalidate_ifs() to invalidate sh.ifstable.

src/cmd/ksh93/tests/posix.sh

@@ -113,6 +113,9 @@ got=$(set --noposix; set $val; echo "$#")
 exp=5
 [[ $got == "$exp" ]] || err_exit "repeated IFS whitespace char (noposix): incorrect number of fields" \
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+got=$(set --default; set $val; echo "$#")
+[[ $got == "$exp" ]] || err_exit "repeated IFS whitespace char (default): incorrect number of fields" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 IFS=$' \t\n' # default
 
 # causes file descriptors > 2 to be left open when invoking another program;