Remember time of last user push activity

Remember time of last user push activity

When a user pushes and successfully passes the initial push
permissions check, update the time stamp on the file
$Girocco::Config:chroot/etc/sshactive/$USER to record the
time of the most recent push activity for $USER.

Despite the name "sshactive" both https and ssh push activity
updates the time stamp on the file.

When combined with a user's creation date, this allows one to
determine whether or not the user has ever actually been used
to push anything in the recent past.

Since fetches do not update this time stamp, it's possible
the user's key was used for fetching, but since all repos are
world-readable there's no reason to keep a user around only
for fetches should inactive user removal be implemented in
the future.