Avoid logging pre-verstring EPIPE from sharing downstreams.

Avoid logging pre-verstring EPIPE from sharing downstreams.

If you use the new 'plink -shareexists' feature, then on Unix at least
it's possible for the upstream to receive EPIPE, because the
downstream makes a test connection and immediately closes it, so that
upstream fails to write its version string.

This looks a bit ugly in the upstream's Event Log, so I'm making a
special case: an error of 'broken pipe' type, which occurs on a socket
from a connection sharing downstream, before we've received a version
string from that downstream, is treated as an unusual kind of normal
connection termination and not logged as an error.