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/arith.c

@@ -511,7 +511,6 @@ static Sfdouble_t arith(const char **ptr, struct lval *lvalue, int type, Sfdoubl
 		}
 		return(r);
 	    }
-
 	    case MESSAGE:
 		sfsync(NIL(Sfio_t*));
 		if(lvalue->emode&ARITH_COMP)
@@ -527,7 +526,6 @@ static Sfdouble_t arith(const char **ptr, struct lval *lvalue, int type, Sfdoubl
  * ptr is set to the last character processed
  * if mode>0, an error will be fatal with value <mode>
  */
-
 Sfdouble_t sh_strnum(register const char *str, char** ptr, int mode)
 {
 	register Sfdouble_t d;