Fix sparse warnings and add an extra sprinkling of const.

This is more or less the equivalent of Sven-Haegar Koch's fixes in the 1.1
branch.

src/protocol.c

@@ -189,7 +189,7 @@ void exit_requests(void) {
 }
 
 bool seen_request(char *request) {
-	past_request_t *new, p = {0};
+	past_request_t *new, p = {NULL};
 
 	p.request = request;