The child process handling PAM authentication can have multiple requests
queued up. As the first success will unlock the screen, there is no
point in processing anything afterwards.
This implements a simpler version of the wlroots logger for swaylock.
With this logger, the dependency on wlroots can be dropped. This also
adds a debug flag and disables debugging output by default
This involves setuid'ing swaylock, which then forks and drops perms on
the parent process. The child process remains root and listens on a pipe
for requests to validate passwords against /etc/shadow.
