Introducing readline function in libunit.

Ruby and Java modules now use this function instead of own
implementations.

src/nxt_unit.h

@@ -335,6 +335,9 @@ int nxt_unit_response_write_cb(nxt_unit_request_info_t *req,
 ssize_t nxt_unit_request_read(nxt_unit_request_info_t *req, void *dst,
     size_t size);
 
+ssize_t nxt_unit_request_readline_size(nxt_unit_request_info_t *req,
+    size_t max_size);
+
 void nxt_unit_request_done(nxt_unit_request_info_t *req, int rc);