More misc. tweaks and cleanups

Notable changes:

.github/workflows/ci.yml:
- Run 'bin/package test' on the github runner so we test iffe too.

src/cmd/ksh93/sh/subshell.c:
- sh_assignok was usually called like 'np = sh_assignok(np,0)'. But
  the function never changes np, it just returns the np value
  passed to it, so the assignment is pointless and that function
  can be changed to a void.

src/cmd/ksh93/sh/fault.c: sh_fault():
- Remove check for sh.subshell after sh_isstate(SH_INTERACTIVE). As
  of 48ba6964, it is never set in subshells.

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         export TZ=UTC
         ulimit -n 1024
         : default regression tests &&
-        script -q -e -c "bin/shtests" &&
+        script -q -e -c "bin/package test" &&
         : regression tests with OS-provided multibyte locales &&
         LANG=nl_NL.UTF-8 script -q -e -c "bin/shtests --locale --nocompile" &&
         LANG=ja_JP.SJIS  script -q -e -c "bin/shtests --locale --nocompile" &&