Thou shalt have no other screen lockers

Multiple screen lockers can interfere in various ways:

- First of all, log spam: only the first locker is usually able to lock at all. Any others will usually fail getting the required input grabs.

- Some lockers, e.g. light-locker, also have specific countermeasures against failing to lock and may interfere with xsecurelock. This has in the past caused issues such as having to unlock twice, having to unlock on a black screen because the graphics output broke, or losing all input focus after unlocking (yes, including alt-tab) and thus having a broken environment until clicking another window.

- Also, potential race conditions during startup of both at the same time might cause deadlocks (e.g. if one grabs mouse first and one grabs keyboard first, while retrying indefinitely).

- Even if they're not retrying indefinitely (xsecurelock isn't), this can slow down suspend to the point that systemd gives up and suspends before the screen is locked.