Make the behavior of the '(' command match the ')' command.

Previously, the ')' command required a sentence to end with two
blanks, but '(' only required a single space.  Historic vi (and the
documentation) requires two spaces in both directions.

Based on a diff from Walter Alejandro Iglesias.

usr.bin/vi/vi/v_sentence.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: v_sentence.c,v 1.12 2026/06/16 02:15:14 millert Exp $	*/
+/*	$OpenBSD: v_sentence.c,v 1.13 2026/06/16 22:59:03 millert Exp $	*/
 
 /*-
  * Copyright (c) 1992, 1993, 1994
@@ -219,7 +219,7 @@ v_sentenceb(SCR *sp, VICMD *vp)
 	recno_t slno;
 	size_t len, scno;
 	u_long cnt;
-	int last;
+	int last, next;
 
 	/*
 	 * !!!
@@ -263,7 +263,7 @@ v_sentenceb(SCR *sp, VICMD *vp)
 				break;
 		}
 
-	for (last = 0;;) {
+	for (last = 0, next = '\0';;) {
 		if (cs_prev(sp, &cs))
 			return (1);
 		if (cs.cs_flags == CS_SOF)	/* SOF is a movement sink. */
@@ -346,7 +346,12 @@ ret:			slno = cs.cs_lno;
 			    cs.cs_flags == CS_EOL || isblank(cs.cs_ch) ||
 			    cs.cs_ch == ')' || cs.cs_ch == ']' ||
 			    cs.cs_ch == '"' || cs.cs_ch == '\'' ? 1 : 0;
+
+			/* A sentence ends with two spaces. */
+			if (cs.cs_ch == ' ' && next != ' ')
+				last = 0;
 		}
+		next = cs.cs_ch;
 	}
 
 okret:	vp->m_stop.lno = cs.cs_lno;