[server] rename httpserver to fileserver

The name "httpserver" is misleading, since it only handles raw file
upload/download.
Merge pull request #715 from haiwen/fserver

[server] rename httpserver to fileserver
[httpserver] bm->size >= 0 is always true

    In struct _BMetadata, size is unsigned.
    No need to check if bm->size >= 0