[httpd/jsonapi] Add cache control headers to some json api endpoints

Adds utility functions to httpd.c for checking the request headers for
either an "If-None-Match" or an "If-Not-Modified-Since" headers. If the
header value is found and it matches the current value for the requested
resource, we return early with a http response code 403 (Not Modified).
If the request header value is not present or does not match we add the
current ETag/Last-Modified values to the response headers and process
the request normally.

src/httpd.h

@@ -4,6 +4,7 @@
 
 #include <stdbool.h>
 #include <regex.h>
+#include <time.h>
 #include <event2/http.h>
 #include <event2/buffer.h>
 #include <event2/keyvalq_struct.h>
@@ -100,6 +101,12 @@ httpd_request_parse(struct evhttp_request *req, struct httpd_uri_parsed *uri_par
 void
 httpd_stream_file(struct evhttp_request *req, int id);
 
+bool
+httpd_request_not_modified_since(struct evhttp_request *req, const time_t *mtime);
+
+bool
+httpd_request_etag_matches(struct evhttp_request *req, const char *etag);
+
 /*
  * Gzips an evbuffer
  *