homed: Allow user to change parts of their record

homed: Allow user to change parts of their record

This allows an unprivileged user that is active at the console to change
the fields that are in the selfModifiable allowlists (introduced in a
previous commit) without authenticating as a system administrator.

Administrators can disable this behavior per-user by setting the
relevant selfModifiable allowlists, or system-wide by changing the
policy of the org.freedesktop.home1.update-home-by-owner Polkit action.