Merge branch 'cb/cvsserver' into maint

"git cvsserver" had a long-standing bug in its authentication code,
which has finally been corrected (it is unclear and is a separate
question if anybody is seriously using it, though).

* cb/cvsserver:
  Documentation: cleanup git-cvsserver
  git-cvsserver: protect against NULL in crypt(3)
  git-cvsserver: use crypt correctly to compare password hashes