mail/exim: daemon fails to create notifier socket on start if no correct exit (+)

Since recent version (e.g. 4.98) exim successfully creates notifier socket file in FreeBSD.
But if file already exists during exim start it's just ignored with error message in log.
Without notifier socket exim cannot check if there is already running exim daemon
with e.g. such condition: ${if eq {$queue_size} {} }.

Add socket file removing before daemon start.
exim_notifier_socket variable is added for notifier_socket setting overriding in exim configuration.

PR: 281615
Notified by: vvd