Previous implementations of "give shell access to some gitolite users"
feature were crap. There was no easy/elegant way to ensure that someone
who had repo admin access would not manage to get himself shell access.
Giving someone shell access requires that you should have shell access
in the first place, so the simplest way is to enable it from the server
side only.
So now that we decided to do that, we may as well prepare for other,
future, commands by starting a server-side utility program with
sub-commands (the only current one being "shell-add")
