MINOR: http: Add standalone functions to parse a start-line or a header

These 2 functions are pretty naive. They only split a start-line into its 3
substrings or a header line into its name and value. Spaces before and after
each part are skipped. No CRLF at the end are expected.
diff --git a/include/common/http.h b/include/common/http.h
index 3b1620d..0835d07 100644
--- a/include/common/http.h
+++ b/include/common/http.h
@@ -153,6 +153,9 @@
                              const char* url_param_name, size_t url_param_name_l,
                              const char **vstart, const char **vend, char delim);
 
+int http_parse_header(const struct ist hdr, struct ist *name, struct ist *value);
+int http_parse_stline(const struct ist line, struct ist *p1, struct ist *p2, struct ist *p3);
+
 /*
  * Given a path string and its length, find the position of beginning of the
  * query string. Returns NULL if no query string is found in the path.