core/socket: we may get ENOTCONN from socket_instantiate_service()

core/socket: we may get ENOTCONN from socket_instantiate_service()

This means that the connection was aborted before we even got to figure out
what the service name will be. Let's treat this as a non-event and close the
connection fd without any further messages.

Code last changed in 934ef6a5.
Reported-by: Thiago Macieira <thiago.macieira@intel.com>
With the patch:
systemd[1]: foobar.socket: Incoming traffic
systemd[1]: foobar.socket: Got ENOTCONN on incoming socket, assuming aborted connection attempt, ignoring.
...

Also, when we get ENOMEM, don't give the hint about missing unit.