units: make systemd-binfmt.service easier to work with no autofs

units: make systemd-binfmt.service easier to work with no autofs

See https://bugzilla.redhat.com/show_bug.cgi?id=1731772:
when autofs4 is disabled in the kernel,
proc-sys-fs-binfmt_misc.automount is not started, so the binfmt_misc module is
never loaded. If we added a dependency on proc-sys-fs-binfmt_misc.mount
to systemd-binfmt.service, things would work even if autofs4 was disabled, but
we would unconditionally pull in the module and mount, which we don't want to do.
(Right now we ony load the module if some binfmt is configured.)
But let's make it easier to handle this case by doing two changes:
1. order systemd-binfmt.service after the .mount unit (so that the .service
   can count on the mount if both units are pulled in, even if .automount
   is skipped)
2. add [Install] section to the service unit. This way the user can do
   'systemctl enable proc-sys-fs-binfmt_misc.mount' to get the appropriate behaviour.