nzbget - add libpar2 as subformula, change gnutls to openssl

As nzbget was the only formula using libpar2, have changed it into a subformula that gets installed to nzbget's libexec. Also added patches to libpar2 from nzbget, as libpar2 upstream hasn't had any activity in years. The patches are bugfixes and add the ability to cancel par2 repair. They are required for the latest nzbget.

Have moved from gnutls to openssl for nzbget, as gnutls was causing issues, and wasn't necessary when OS X provides openssl. Have tested with openssl, no issues found, and no errors that gnutls was exhibiting.

This is my first subformula, and my first proper edit of a formula. If I've messed something up, please let me know so I can fix it!