BUG/MEDIUM: unique_id: junk in log on empty unique_id

When a request fail, the unique_id was allocated but not generated.
The string was not initialized and junk was printed in the log with %ID.

This patch changes the behavior of the unique_id. The unique_id is now
generated when a request failed.

This bug was reported by Patrick Hemmer.
BUG/MINOR: log: junk at the end of syslog packet

With a facily of 2 or 1 digit, the send size was wrong and bytes with
unknown value were sent.
The size was calculated using the start of the buffer and not the start
of the data which varies with the number of digits of the facility.

This bug was reported by Samuel Stoller and reported by Lukas Tribus.
DOC: add a mention about the limited chunk size

We now indicate that PD flags can be returned for chunk sizes >= 2GB.
MINOR: ssl: Add statement 'verifyhost' to "server" statements

verifyhost allows you to specify a hostname that the remote server's
SSL certificate must match. Connections that don't match will be
closed with an SSL error.
BUG/MEDIUM: fix broken send_proxy on FreeBSD

David Berard reported that send-proxy was broken on FreeBSD and tracked the
issue to be an error returned by send(). We already had the same issue in
the past in another area which was addressed by the following commit :

   0ea0cf6 BUG: raw_sock: also consider ENOTCONN in addition to EAGAIN

In fact, on Linux send() returns EAGAIN when the connection is not yet
established while other OSes return ENOTCONN. Let's consider ENOTCONN for
send-proxy there as the same as EAGAIN.

David confirmed that this change properly fixed the issue.

Another place was affected as well (health checks with send-proxy), and
was fixed.

This fix does not need any backport since it only affects 1.5.