gnupg2: port "8192bit key support" option from gnupg forumla.

see:
* mxcl/homebrew#4201
* https://github.com/mxcl/homebrew/commit/d092d71645593e1f2a0e1d5f892be7b368b07d98
* [current gnupg formula](https://github.com/mxcl/homebrew/blob/master/Library/Formula/gnupg.rb)

`g10/keygen.c` is identical or near-identical between gnupg and gnupg2, so porting the option & inreplace works exactly the same.