Add --no-http-keepalive option to help work around Cisco incompetence.

Add --no-http-keepalive option to help work around Cisco incompetence.

We know that certain versions of the ASA software (8.2.2.5 at least) are
buggy and will 'forget' the client's SSL certificate by the time they
receive the second request on a re-used HTTP connection. We have an
unconditional workaround for the case where we _know_ that bug will
trip, in commit 357c85e8 ("Always close HTTP/1.0 connection...").

Cisco's support staff are completely useless and have failed to give any
competent response to the bug report -- so not only does it look like
they won't fix it, but we don't actually know what under _other_
circumstances this same bug might manifest itself.

This patch adds an option to disable _all_ connection re-use. The
intention is that users can try it out if they encounter problems, then
report to the mailing list that it worked so that we can work out how
to trigger it automatically.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>