sh: Don't check input for non-whitespace if history is disabled.

preadbuffer() maintained a flag whether there was any non-whitespace
character. This flag is only useful when history is enabled (in that case,
lines containing only whitespace are not added to history). Instead, check
using strspn() when history is enabled.

There is an approximate 2% speedup when running
  sh -c '. /etc/rc.subr; . /etc/defaults/rc.conf; source_rc_confs'
with hot cache.

bin/sh/input.c

@@ -228,7 +228,6 @@ preadbuffer(void)
 {
 	char *p, *q;
 	int more;
-	int something;
 	char savec;
 
 	if (parsefile->strpush) {
@@ -252,24 +251,18 @@ preadbuffer(void)
 	q = p = parsefile->buf + (parsenextc - parsefile->buf);
 
 	/* delete nul characters */
-	something = 0;
 	for (more = 1; more;) {
 		switch (*p) {
 		case '\0':
 			p++;	/* Skip nul */
 			goto check;
 
-		case '\t':
-		case ' ':
-			break;
-
 		case '\n':
 			parsenleft = q - parsenextc;
 			more = 0; /* Stop processing here */
 			break;
 
 		default:
-			something = 1;
 			break;
 		}
 
@@ -288,7 +281,8 @@ preadbuffer(void)
 	*q = '\0';
 
 #ifndef NO_HISTORY
-	if (parsefile->fd == 0 && hist && something) {
+	if (parsefile->fd == 0 && hist &&
+	    parsenextc[strspn(parsenextc, " \t\n")] != '\0') {
 		HistEvent he;
 		INTOFF;
 		history(hist, &he, whichprompt == 1 ? H_ENTER : H_ADD,