allow to change local files

- fuse/filecache:
     * add filecache_upload_patch
     * allow opening files in modes other than RDONLY
 - fuse/hashtbl:
     * add folder_tree_upload_patch
 - fuse/operations:
     * allow opening files in modes other than RDONLY
     * add members to private context which allow tracking of
       not-yet-uploaded files and files opened for writing and
       files opened in read-only mode
 - mfapi/apicalls/upload_patch:
     * supply x-filename and x-filesize headers
 - mfapi/apicalls/upload_simple:
     * do not supply the x-filehash header as it is not used by the
       server
 - utils/hash:
     * hex characters must be lower case for the server
 - utils/strings:
     * clean up unused functions strdup_join, strdup_substr,
       string_chomp
 - utils/stringv:
     * complete rewrite with different string vector implementation

utils/http.c

@@ -289,6 +289,7 @@ http_post_file(mfhttp * conn, const char *url, FILE * fh,
 
     if (custom_headers == NULL) {
         custom_headers = &fallback_headers;
+        *custom_headers = NULL;
     }
     // when using POST, curl implicitly sets
     // Content-Type: application/x-www-form-urlencoded