security/libressl: Add the possibility to build only libtls

[David Wahlund]

Hi
I'd like to use the libtls library of LibreSSL on FreeBSD. Or the python 
bindings to libtls specifically. I do NOT however want to replace 
openssl or use the libssl library.

 From what I understand it would be possible in practice as I assume 
it's only libssl that overwrites files used by openssl.

Would it be possible to create an option in LibreSSL, or preferably make 
a separate port, for libtls only? That way future ports can depend on 
libtls only. For example a future python-libtls port could depend on that.

-David