More fixes for subshell directory handling (re: 7bab9508, 5ee290c7)

This commit fixes what are hopefully the two final aspects of #153:

1. If the present working directory does not exist (was moved or
   deleted) upon entering a virtual subshell, no PWD directory path
   is saved. Since restoring the state after exiting a virtual
   subshell is contingent on a previous PWD path existing, this
   resulted in entire aspects of the virtual subshell, such as the
   subshell function tree, not being cleaned up.
2. A separate problem is that 'cd ..' does not update PWD or OLDPWD
   when run from a nonexistent directory.

A reproducer exposing both problems is:

$ mkdir test
$ cd test
$ ksh -c '(subfn() { BAD; }; cd ..; echo subPWD==$PWD);
			typeset -f subfn; echo mainPWD==$PWD'
subPWD==/usr/local/src/ksh93/ksh/test
subfn() { BAD; };mainPWD==/usr/local/src/ksh93/ksh/test

Expected output:
subPWD==/usr/local/src/ksh93/ksh
mainPWD==/usr/local/src/ksh93/ksh/test

src/cmd/ksh93/bltins/cd_pwd.c:
- If path_pwd() fails to get the PWD (usually it no longer exists),
  don't set $OLDPWD to '.' as that is pointless; use $PWD instead.
  After cd'ing from a nonexistent directory, 'cd -' *should* fail
  and should not be equivalent to 'cd .'.
- Remove a redundant check for (!oldpwd) where it is always set.
- Do not prematurely return without setting PWD or OLDPWD if
  pathcanon() fails to canonicalise a nonexistent directory.
  Instead, fall back to setting PWD to the result of getcwd(3).

src/cmd/ksh93/sh/subshell.c:
- Minor stylistic adjustment. Some NULL macros sneaked in. This
  historic code base does not use them (yet); change to NIL(type*).
- sh_subshell(): Fix logic for determining whether to save/restore
  subshell state.
  1. When saving, 'if(!comsub || !shp->subshare)' is redundant;
     'if(!shp->subshare)' should be enough. If we're not in a
     subshare, state should be saved.
  2. When restoring, 'if(sp->shpwd)' is just nonsense as there is
     no guarantee that the PWD exists upon entering a subshell.
     Simply use the same 'if(!shp->subshare)'. Add an extra check
     for sp->pwd to avoid a possible segfault. Always restore the
     PWD on subshell exit and not only if shp->pwd is set.
- sh_subshell(): Issue fatal errors in libast's "panic" format.

src/cmd/ksh93/tests/builtins.sh:
- Adjust a relevant test to run err_exit() outside of the subshell
  so that any error is counted in the main shell.
- Add test for problem 2 described at the top.

src/cmd/ksh93/tests/subshell.sh:
- Add test for problems 1 and 2 based on reproducer above.

Resolves: https://github.com/ksh93/ksh/issues/153

src/cmd/ksh93/tests/subshell.sh

@@ -887,17 +887,21 @@ got=$( { "$SHELL" "$tmp/crash_rhbz1117404.ksh"; } 2>&1)
 # ======
 # Segmentation fault when using cd in a subshell, when current directory cannot be determined
 # https://github.com/ksh93/ksh/issues/153
-cd "$tmp"
-mkdir deleted
-cd deleted
+mkdir "$tmp/deleted"
+cd "$tmp/deleted"
 tmp=$tmp "$SHELL" -c 'cd /; rmdir "$tmp/deleted"'
+
+exp="subPWD: ${PWD%/deleted}"$'\n'"mainPWD: $PWD"
+got=$( { "$SHELL" -c '(subshfn() { bad; }; cd ..; echo "subPWD: $PWD"); typeset -f subshfn; echo "mainPWD: $PWD"'; } 2>&1 )
+[[ $got == "$exp" ]] || err_exit "subshell state not restored after 'cd ..' from deleted PWD" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 exp="PWD=$PWD"
 got=$( { "$SHELL" -c '(cd /; (cd /)); print -r -- "PWD=$PWD"'; } 2>&1 )
 ((!(e = $?))) && [[ $got == "$exp" ]] || err_exit 'failed to restore nonexistent PWD on exiting a virtual subshell' \
 	"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
-cd "$tmp"
-mkdir recreated
-cd recreated
+mkdir "$tmp/recreated"
+cd "$tmp/recreated"
 tmp=$tmp "$SHELL" -c 'cd /; rmdir "$tmp/recreated"; mkdir "$tmp/recreated"'
 exp="PWD=$PWD"
 got=$( { "$SHELL" -c '(cd /); print -r -- "PWD=$PWD"'; } 2>&1 )