Configure gnupg to use pinentry by default.

This makes gnupg much more usable out of the box.
Remove obsolete patch.
Remove use* options and let the user set optional dependies to null if
they don't want to use them.
Apply recent changes from gnupg/default.nix to gnupg/git.nix

gnupg/git.nix doesn't build, but it didn't build before either.
Add roconnor to GnuPG maintainers.
Remove unmaintained gnupg2_1.
Comment on optional dependencies for Gnupg.
Merge pull request #4120 from NixOS/gnupg

Add pinentry optional dependency for gnupg.

Delete gnupg2_1.