Fix #socket_state.sockmod to use ejabberd_tls

e6508cab renamed `tls.erl` to `ejabberd_tls.erl` in order to avoid a
conflict with the `tls` module (from `ssl` application) introduced in R16.
This commit fixes some places which were still referring to `tls` instead
of `ejabberd_tls`.
Merge pull request #99 from esl/fix-tls-to-ejabberd-tls

Fix #socket_state.sockmod to use ejabberd_tls