recognize + as per manual page

Reference: /n/atom/patch/applied2013/upasflags
Date: Sat Jul 20 00:10:13 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/common/flags.c	Sat Jul 20 00:10:05 2013
+++ /sys/src/cmd/upas/common/flags.c	Sat Jul 20 00:10:05 2013
@@ -47,9 +47,13 @@
 
 	for(;;){
 		neg = 0;
+	again:
 		if((c = *p++) == '-'){
 			neg = 1;
-			c = *p++;
+			goto again;
+		}else if(c == '+'){
+			neg = 0;
+			goto again;	
 		}
 		if(c == 0)
 			return nil;