https://github.com/NixOS/nixpkgs/pull/107497 broke booting on many systems that
use tmpOnTmpfs due to the lack of specifying the mount type.
This commit explicitly adds the mount type, which should fix booting
such systems.
The original change may want to be revisited however too.
@poettering decided we only need a limited number of inodes in our /tmp,
so why not limit that for every systemd user? That makes medium-sized nix
builds impossible so this commit restores the old behaviour which is the
kernel default of half the number of physical RAM pages which does not
seem too unreasonable to me.
/tmp cleaning is done by systemd rather than stage-2-init
enableEmergencyMode moved from systemd to seperate module
new option to mount tmp on tmpfs
new option to enable additional units shipped with systemd
