Another round of accumulated minor fixes and cleanups

Only notable changes listed below.

**/Mamfile:
- Do not bother redirecting standard error for 'cmp -s' to
  /dev/null. Normally, 'cmp -s' on Linux, macOS, *BSD, or Solaris
  do not not print any message. If it does, something unusual is
  going on and I would want to see the message.
- Since we now require a POSIX shell, we can use '!'.

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/sh/init.c:
- Remove SH_TYPE_PROFILE symbol, unused after the removal of the
  SHOPT_PFSH code. (re: eabd6453)

src/cmd/ksh93/sh/io.c:
- piperead(), slowread(): Replace redundant sffileno() calls by
  the variables already containing their results. (re: 50d342e4)

src/cmd/ksh93/bltins/mkservice.c,
rc/lib/libcmd/vmstate.c:
- If these aren't compiled, define a stub function to silence the
  ranlib(1) warning that the .o file does not contain symbols.

src/cmd/ksh93/sh/fault.c

@@ -380,7 +380,6 @@ void	sh_sigclear(register int sig)
 /*
  * check for traps
  */
-
 void	sh_chktrap(void)
 {
 	register int 	sig=sh.st.trapmax;
@@ -606,7 +605,6 @@ static void array_notify(Namval_t *np, void *data)
 /*
  * This is the exit routine for the shell
  */
-
 noreturn void sh_done(register int sig)
 {
 	register char *t;