[Exim] keepalive documentation

In the 3.30 docs, the description for smtp_accept_keepalive reads:
>This option controls the setting of the SO_KEEPALIVE option on incoming
>TCP/IP socket connections. This causes the kernel periodically to send
>some OOB (out-of-band) data on idle connections.

This description is confusing since the term OOB has been abused by
the BSD API to mean 'set the urgent flag'. [See the MSG_OOB flag on
send(2).]

Here's some replacement text for the smtp_accept_keepalive option that
makes it clearer what the option does. It is also applicable to the
keepalive option in the smtp driver.

"This option controls the setting of the SO_KEEPALIVE option on
incoming TCP/IP socket connections. This causes the kernel to probe
idle connections by sending packets with an `old' sequence
numbers. The other end of the connection should send a acknowledgement
if the connection is still okay or a reset is the connection has been
aborted. The keepalive mechanism takes several hours to detect
unreachable hosts.

The reason for doing this is that it has the beneficial effect of
freeing up certain types of connection that can get stuck when the
remote host is disconnected without tidying up the TCP/IP call
properly."

Peter Benie