[PATCH] Log details for client certificate failures

Hello,

(I'm cleaning up some old git branches and found this. It was helpful
when I was trying to debug failures between an NSS client and an
OpenSSL server, and it seems general enough to help for more
complicated OpenSSL-only setups as well.)

Currently, debugging client cert verification failures is mostly
limited to looking at the TLS alert code on the client side. For simple
deployments, usually it's enough to see "sslv3 alert certificate
revoked" and know exactly what needs to be fixed, but if you add any
more complexity (multiple CA layers, misconfigured CA certificates,
etc.), trying to debug what happened based on the TLS alert alone can
be an exercise in frustration.

Luckily, the server has more information about exactly what failed in
the chain, and we already have the requisite callback implemented as a
stub, so I've filled it out with error handling and added a COMMERROR
log so that a DBA can debug client failures more easily.

It ends up looking like

    LOG:  connection received: host=localhost port=44120
    LOG:  client certificate verification failed at depth 1: unable to get local issuer certificate
    DETAIL:  failed certificate's subject: /CN=Test CA for PostgreSQL SSL regression test client certs
    LOG:  could not accept SSL connection: certificate verify failed

It might be even nicer to make this available to the client, but I
think the server log is an appropriate place for this information -- an
admin might not want to advertise exactly why a client certificate has
failed verification (other than what's already available via the TLS
alert, that is), and I think more complicated failures (with
intermediate CAs, etc.) are going to need administrator intervention
anyway. So having to check the logs doesn't seem like a big hurdle.

One question/concern -- the Subject that's printed to the logs could be
pretty big (OpenSSL limits the incoming certificate chain to 100K, by
default), which introduces an avenue for intentional log spamming. Is
there an existing convention for limiting the length of log output used
for debugging? Maybe I should just hardcode a smaller limit and
truncate anything past that? Or we could just log the Common Name,
which should be limited to 64 bytes...

I'll add this to the July commitfest.

Thanks,
--Jacob