Log Message:
ffmpeg[234]: Add an ssl optional group to only choice gnutls or openssl option

At least multimedia/ffmpeg4 does not support to be built with both `gnutls' and
`openssl'.  Introduce an `ssl' optional group in order to select only `gnutls'
or `openssl' option.

Noticed by John D. Baker on pkgsrc-users@:

 <https://mail-index.NetBSD.org/pkgsrc-users/2018/09/25/msg027482.html>

Thanks to John for spotting that and <gdt> for review!