command -x: fix 'arg list too long' on Linux (re: 66e1d44)

I got one intermittent regression test failure due to 'argument
list too long' on a Debian x86_64 system.

src/cmd/ksh93/sh/path.c: path_xargs():
- Leave extra argument space for systems that need extra bytes:
  1KiB per extra byte, with a minimum of 2KiB (the old value).

src/cmd/ksh93/sh/path.c

@@ -162,7 +162,7 @@ static pid_t path_xargs(Shell_t *shp,const char *path, char *argv[],char *const
 	pid_t pid;
 	if(shp->xargmin < 0)
 		return((pid_t)-1);
-	size = shp->gd->lim.arg_max-2048;
+	size = shp->gd->lim.arg_max - (ARG_EXTRA_BYTES > 2 ? 1024*ARG_EXTRA_BYTES : 2048);
 	for(ev=envp; cp= *ev; ev++)
 		size -= strlen(cp) + 1 + ARG_EXTRA_BYTES;
 	for(av=argv; (cp= *av) && av< &argv[shp->xargmin]; av++)