cleanup: removed last references to globals from client

- There were some refs in SyntheticClient to g_(ceph_context|conf), I
  replaced them with client->cct and client->cct->_conf.
- There were some refs in fuse_ll to g_conf, also replaced them with
  client->cct or cfuse->client->cct where applicable.

This makes everything in src/client completely independent from globals.

Signed-off-by: Roald J. van Loon <roaldvanloon@gmail.com>