By default systemd kills the service which causes an OOM. That make
sense for a typical service, however, for SSH we don't want this
behavior: The connection should continue, just the command which caused
OOM should be killed.
This can be helpful when debugging HAOS issues. Dropbear is only started
for users which actually enabled it by configuring a SSH key, so this
change won't have an effect for most people.
So far the exit code has been evaluated, which seems to be non-zero even
with a regular term signal. With that systemd assumed the service is in
a failed state, when in fact this seems the regular behavior of dropbear
when shutting it down.
