fuser: Fix skipping "SIG" on signal names (-s).

The code did  !strncasecmp(str, "sig", 4)  which is not useful.

Also change "sig" to "SIG" matching the uppercase signal names as of
r218285. This has little effect because fuser does not enable locale.

usr.bin/fstat/fuser.c

@@ -358,9 +358,8 @@ str2sig(const char *str)
 {
 	int i;
 
-#define	SIGPREFIX	"sig"
-	if (!strncasecmp(str, SIGPREFIX, sizeof(SIGPREFIX)))
-		str += sizeof(SIGPREFIX);
+	if (!strncasecmp(str, "SIG", 3))
+		str += 3;
 	for (i = 1; i < sys_nsig; i++) {
                 if (!strcasecmp(sys_signame[i], str))
                         return (i);