Log Message:
Address issues raised by agc@ to make the github master site handling
more pkgsrcesque.

1) turn _USE_GITHUB into an internal infrastructure variable
2) put back MASTER_SITES in packages' Makefiles.
3) encode the account in the master site URL, remove GH_ACCOUNT
4) rename GH_PROJECT to GITHUB_PROJECT
5) rename GH_TAGNAME to GITHUB_TAG and allow it to accept commit hash
   as well as tag. GH_COMMIT is gone.
6) turn on this functionality when MASTER_SITES matches a predefined github
   pattern instead of via explicit USE_GITHUB setting.