Continue trying to grab the keyboard until we succeed in doing so

If other grabbed keys (for example Super+L) are being pressed as we start up,
the XGrabKeyboard may fail.  For this reason, we need to try again until we
succeed.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Added support for ~/.cantera/lock-passkey

This allows having a different password for screen locking than for login, or
using the application with no visible password hash in /etc/passwd

Signed-off-by: Morten Hustveit <mortehu@google.com>
Added genpwhash for generating crypt hashes

This is useful in conjunction with ~/.cantera/lock-passkey

Signed-off-by: Morten Hustveit <mortehu@google.com>
Removed unused dependency on libXxf86vm