Return the servers when asked to do so

Return the elarm_server children - not an empty list (as it does now),
not a list containing the elarm_registry (as a badly written patch
could do).
Add unit tests re which_servers
Fix typo
Add additional supervision level on top of elarm servers...

... in order to move the 'which_servers' function down from elarm_sup,
as elarm_registry:init cannot invoke elarm_sup:which_servers - or it
gets stuck.

No externally-visible functional changes yet (this change prepares the
ground for a following change).
Fetch current list of servers when elarm_registry restarts after crash

The change in elarm_man_sup_sup is needed in order to prevent that
elarm_registry, while starting, waits indefinitely for
elarm_man_sup_sup not started yet (or being restarted).

Fixes https://github.com/jonasrichard/elarm/commit/c69b05840ad9c0dabd9f83b554fdc690cc96a710#commitcomment-6459372

Implements https://github.com/jonasrichard/elarm/commit/c69b05840ad9c0dabd9f83b554fdc690cc96a710#commitcomment-6489435
(implements change to elarm_registry:init *and* the supervision tree
in order to avoid cycle dependency on elarm_sup)