Introduce elogind-uaccess-command to replace uaccess builtin.

Introduce elogind-uaccess-command to replace uaccess builtin.

The uaccess udev builtin command is only used by logind and contains
functionality only implemented in logind. As such, while we cannot
write udev-builtin commands in elogind (not being udev), we can write
standalone binaries and rewrite our udev rules to use them instead.

This fixes the feature of granting users access to devices using a user
ACL which is toggled only when the user is associated with an active
session. Currently this functionality is half broken, as while the ACL
is granted and revoked while VT-switching, it is not granted to new
devices as they are plugged in. This issue is fixed by this commit.