fix Cbegin constant

Reference: /n/atom/patch/applied/grepcbegin
Date: Sun Mar 30 01:16:25 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/grep/grep.h	Sun Mar 30 01:16:20 2014
+++ /sys/src/cmd/grep/grep.h	Sun Mar 30 01:16:21 2014
@@ -53,7 +53,7 @@
 
 	Caselim		= 7,
 	Nhunk		= 1<<16,
-	Cbegin		= 0x10000,
+	Cbegin		= Runemax+1,
 	Flshcnt		= (1<<9)-1,
 
 	Cflag		= 1<<0,