sysusers: allow overrides in /etc and /run

An administrator might want to block a certain sysusers config file from
being executed, e.g. to block the creation of a certain user.

Only a relatively short description is added in the man page, since
overrides should be relatively rare.
man: proper link for dmesg
man: document x-systemd.device-timeout for crypttab

https://bugs.freedesktop.org/show_bug.cgi?id=54210