command -x: tweak args list size detection (re: 9ddb45b1)

src/cmd/ksh93/features/externs: ARG_EXTRA_BYTES detection:
- Improve detection of extra bytes per argument: on every loop
  iteration, recalculate the size of the environment while taking
  the amount extra bytes we're currently trying into account. Also
  count arguments (argv[]) as they are stored in the same buffer.
  On 64-bit Linux with glibc, this now detects 9 extra bytes per
  argument instead of 8. An odd number (literally and figuratively)
  but apparently it needs it; I do think my method is correct now.
  On 64-bit Solaris and macOS, this still detects 8 extra bytes.
  (On 64-bit Linux with musl C library, it detects 0 bytes. Nice.)

src/cmd/ksh93/sh/path.c: path_xargs():
- Remove the kludge subtracting twice the size of the environment.
  With the feature test fixed, this should no longer fail on Linux.
- Take into account the size of the final null element in the
  argument and environment lists.

src/cmd/ksh93/tests/path.sh:
- Do not use awk for the test due to breakage in the system awks
  on Solaris/Illumos (hangs) and AIX & UnixWare (drops arguments).
  Instead, use (wait for it...) ksh. It's a bit slower, but works.

src/cmd/ksh93/sh/path.c

@@ -164,11 +164,12 @@ static pid_t path_xargs(Shell_t *shp,const char *path, char *argv[],char *const
 		return((pid_t)-1);
 	size = shp->gd->lim.arg_max - (ARG_EXTRA_BYTES > 2 ? 1024*ARG_EXTRA_BYTES : 2048);
 	for(ev=envp; cp= *ev; ev++)
-		size -= 2 * (strlen(cp) + 1 + ARG_EXTRA_BYTES);
+		size -= strlen(cp) + 1 + ARG_EXTRA_BYTES;
 	for(av=argv; (cp= *av) && av< &argv[shp->xargmin]; av++)  
 		size -= strlen(cp) + 1 + ARG_EXTRA_BYTES;
 	for(av=avlast; cp= *av; av++,nlast++)  
 		size -= strlen(cp) + 1 + ARG_EXTRA_BYTES;
+	size -= 2 + 2 * ARG_EXTRA_BYTES;  /* final null env and arg elements */
 	av =  &argv[shp->xargmin];
 	if(!spawn)
 		job_clear();