Port of most of the gvfs core, few bits still missing. Lot of debug prints around,
will be removed in further commits. Same amount of TODOs and FIXMEs.

Notes:
 * kill serials?
 * get rid of mainloops where applicable (copy/move progress callback, enumerator)
 * fix keyring integration
 * use gdbus builtin fd passing within gvfsdaemon.c, kill the extra_fd stuff