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/main.c

@@ -85,7 +85,6 @@ static char	beenhere = 0;
  * search for file and exfile() it if it exists
  * 1 returned if file found, 0 otherwise
  */
-
 static int sh_source(Sfio_t *iop, const char *file)
 {
 	char*	oid;
@@ -373,7 +372,6 @@ int sh_main(int ac, char *av[], Shinit_f userinit)
  * iop is not null when the input is a string
  * fdin is the input file descriptor 
  */
-
 static void	exfile(register Sfio_t *iop,register int fno)
 {
 	time_t curtime;