README: CONFIG_DEVPTS_MULTIPLE_INSTANCES is necessary

https://bugs.debian.org/773932
bus: replace ENOSYS return codes with EBADR/ENOTSUP

ENOSYS is used to signify compiled-out functionality. Using it for
different kinds of error is misleading.

For BUS_ERROR_SLEEP_VERB_NOT_SUPPORTED, logind-action.c uses ENOTSUP
already, so changing it to ENOTSUP makes the dbus and action paths
behave the same.
systemctl: do not repeat hibernate/sleep attempts

If some sleep operation was not possible (e.g. because swap is missing),
we would try twice: once through logind, which would result in a clean error:
  Failed to execute operation: Sleep verb not supported
and then second time by starting the appropriate unit directly, which is
more messy. If logind tells us that something is not possible (or already
in progress), report that to the user and quit. If logind is present and working
we should not try to work around it.

Loosely based on https://bugs.freedesktop.org/show_bug.cgi?id=87832.