sslmode patch

Folks,

At long last I put together a patch to support 4 client SSL negotiation
modes (and replace the requiressl boolean). The four options were first
spelled out by Magnus Hagander <mha(at)sollentuna(dot)net> on 2000-08-23 in email
to pgsql-hackers, archived here:

http://archives.postgresql.org/pgsql-hackers/2000-08/msg00639.php

My original less-flexible patch and the ensuing thread are archived at:

http://dbforums.com/t623845.html

Attached is a new patch, including documentation.

To sum up, there's a new client parameter "sslmode" and environment
variable "PGSSLMODE", with these options:

sslmode description
------- -----------
prevent Unencrypted non-SSL only
allow Negotiate, prefer non-SSL
prefer Negotiate, prefer SSL (default)
require Require SSL

The only change to the server is a new pg_hba.conf line type,
"hostnossl", for specifying connections that are not allowed to use SSL
(for example, to prevent servers on a local network from accidentally
using SSL and wasting cycles). Thus the 3 pg_hba.conf line types are:

pg_hba.conf line types
----------------------
host applies to either SSL or regular connections
hostssl applies only to SSL connections
hostnossl applies only to regular connections

These client and server options, the postgresql.conf ssl = false option,
and finally the possibility of compiling with no SSL support at all,
make quite a range of combinations to test. I threw together a test
script to try many of them out. It's in a separate tarball with its
config files, a patch to psql so it'll announce SSL connections even in
absence of a tty, and the test output. The test is especially informative
when run on the same tty the postmaster was started on, so the FATAL:
errors during negotiation are interleaved with the psql client output.

I saw Tom write that new submissions for 7.4 have to be in before midnight
local time, and since I'm on the east coast in the US, this just makes it
in before the bell. :)

Jon