Make PATH properly survive a shared-state ${ comsub; }

Reproducer:

$ ksh -c 'v=${ PATH=/dev/null; }; echo $PATH; whence ls'
/dev/null
/bin/ls

The PATH=/dev/null assignment should survive the shared-state
command substitution, and does, yet 'ls' is still found.
The variable became inconsistent with the internal pathlist.

This bugfix is from the 93v- beta.

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- Do not save and restore pathlist for a subshare.
- A few other subshell tweaks from 93v- that made sense:
  . reset shp->subdup (bitmask for dups of 1) after saving it
  . use e_dot instead of "." for consistency
  . retry close(1) if it was interrupted

src/cmd/ksh93/tests/path.sh:
- Add test for this bug.

src/cmd/ksh93/tests/path.sh

@@ -731,6 +731,15 @@ got=$({ FPATH=$tmp/fun.$$ "$SHELL" -c self; } 2>&1)
 (((e = $?) == 126)) || err_exit 'Function autoload recursion loop:' \
 	"got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got")"
 
+# ======
+# If a shared-state ${ command substitution; } changed the value of $PATH, the variable
+# would change but the internal pathlist would not, making path searches inconsistent.
+savePATH=$PATH
+got=${ PATH=/dev/null; }
+got=$(whence ls)
+PATH=$savePATH
+[[ -z $got ]] || err_exit "PATH search inconsistent after changing PATH in subshare (got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))