alias: Avoid unnecessary forks (re: ec888867) (#417)

The code used to fork subshells when creating/changing aliases will
always fork, even when the alias tree isn't changed:
   $ echo $(unalias --man 2> /dev/null; echo $$ ${.sh.pid})
   375097 375107
   $ alias foo=bar; echo $(alias -p foo; echo $$ ${.sh.pid})
   alias foo=bar 375097 375110
This is a bit inefficient, so this commit avoids forking a subshell
unless at least one change is made to the alias table.

src/cmd/ksh93/bltins/typeset.c:
- b_alias(), b_unalias(): Remove sh_subfork() calls.
- setall(): When creating an alias (name contains '='), fork a
  virtual subshell before calling nv_open() to add the node.
- unall():
  - When unsetting all aliases (-a), fork subshell before dtclear().
  - When unsetting one alias, fork subshell before nv_delete().
  - Move sh_pushcontext() and sh_popcontext() expansions so that
    sh_subfork() is not in between them, as that would cause
    program flow corruption or a crash.

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

src/cmd/ksh93/tests/alias.sh

@@ -275,5 +275,23 @@ ret=$?
 got=$?
 ((got > 0)) || err_exit "Exit status is zero when alias is passed 256 non-existent aliases"
 
+# ======
+# https://github.com/ksh93/ksh/pull/417
+
+alias foo=bar
+(unalias -a)
+alias foo >/dev/null 2>&1 || err_exit "unalias -a leaked out of subshell"
+unalias foo
+(alias foo=bar)
+alias foo >/dev/null 2>&1 && err_exit "alias leaked out of subshell"
+
+alias foo=bar
+exp="0 $$"
+got=$(alias foo='echo "$? $$"'; eval foo)
+[[ $got == "$exp" ]] || err_exit "alias in subshell: expected $(printf %q "$exp"), got $(printf %q "$got")"
+got=$(unalias foo; echo "$? $$")
+[[ $got == "$exp" ]] || err_exit "unalias in subshell: expected $(printf %q "$exp"), got $(printf %q "$got")"
+unalias foo
+
 # ======
 exit $((Errors<125?Errors:125))