Events: support for SO_SNDBUF on outbound peer connections

Events: support for SO_SNDBUF on outbound peer connections

ngx_event_connect_peer() honors SO_RCVBUF via pc->rcvbuf but had
no equivalent for SO_SNDBUF.  This adds pc->sndbuf and the matching
setsockopt() call, mirroring the existing SO_RCVBUF path.
Both setsockopt() calls are made non-fatal.

Existing callers leaving sndbuf at 0 retain prior behavior, since
setsockopt() is only invoked when the field is non-zero.  These
fields are consumed by various proxying modules in follow-up commits.