make grep's path adjustable for PANICTRACE + more bits

Mac OS X doesn't have /bin/grep, only /usr/bin/grep.  Grr.
Don't try to parse PANICTRACE_GLIBC if the field doesn't exist.

include/sys.h

@@ -23,6 +23,7 @@ struct sysopt {
 #ifdef PANICTRACE
 		/* panic options */
 	char *gdbpath;
+	char *greppath;
 	int  panictrace_gdb;
 # ifdef PANICTRACE_GLIBC
 	int panictrace_glibc;