saver_blank: Do not get killed during DPMS activation.

Previously, the xset command sometimes got killed in response to
ScreenSaverNotify.
main: Provide sensible defaults for saver and auth modules.

These can be overridden via:
- ./configure --with-default-auth-module=... --with-default-saver-module=...
- $XSECURELOCK_AUTH and $XSECURELOCK_SAVER environment variables
- Command line arguments (which work exactly as before)
auth_pam_x11: Always show the cursor while typing; time out after 60 sec.
main: Always reinstate grabs 10 times per second.

It's sad that this is required, but some XScreenSaver hacks cause
XSecureLock to lose the grab on MotionNotify.