log session end - again

This patch brings up to date what I did last year (now unfortunately
bitrotted) to allow the logging of the end of a session, enabled by the
config setting "log_session_end - true". It produces lines like these:

for a local connection:
[21340] LOG: session end: duration: 0:00:23.41 user=andrew
database=template1 host=[local]

for a tcp connection:
[21351] LOG: session end: duration: 0:00:20.22 user=andrew
database=template1 host=127.0.0.1 port=33419

for a tcp connection when log_hostname = true:
[21373] LOG: session end: duration: 0:00:02.25 user=andrew
database=template1 host=alphonso port=33421

enjoy

andrew