Adding Rustls to Postgres

See NOTES.md for my day-to-day notes.

There are six functions in the client and six functions in the server that we need to implement - see NOTES.md for details.

Implementation steps

1. Get the Postgres frontend compiling with stub functions that don't do anything. The backend will still compile using NSS (ie we'll compile in both libraries until we have them both working.)

2. Try to implement the functions.

3. Get it working on other operating systems, add docs, etc.

I'm going to push my changes to github.com/kevinburke/postgres.

Getting the code up and running

The first thing you want to do is apply Daniel Gustafsson's NSS patches to tip of the upstream project.

0. Install prerequisites - we bootstrap using NSS, so you'll need a valid check out of that for now.

brew install nspr nss

Clone github.com/rustls/rustls-ffi and run:

```
make clean && make && make install DESTDIR='./target'
```

1. Check out github.com/postgres/postgres and (as of October 29) pull the latest commit on the master branch.

2. Find the latest patchset in patchsets here or download from the mailing list.

3. Apply each patch from the NSS patchset:

   for patch in ../../kevinburke/postgres-rustls/patchsets/2021-10-29-gustafsson-mailing-list/*.patch; do git am $patch; done;

4. Apply the latest changes from the rustls-4 branch on the github.com/kevinburke/postgres remote, on top of this patch, rebasing down to one commit if you need.

5. Run "make clean" to clear out any artifacts that may have been remaining from the previous build

6. Compile, make, install. This will install binaries into $HOME/pq. Note you will have to change some of these paths - the ones that say e.g. /Users/kevin.

LDFLAGS="-L/usr/local/opt/nss/lib -L/usr/local/opt/nspr/lib -L/Users/kevin/src/github.com/rustls/rustls-ffi/target/lib" \
    CFLAGS="-framework Security" \
    CPPFLAGS="-framework Security -I/usr/local/opt/nss/include -I/usr/local/opt/nspr/include -I/Users/kevin/src/github.com/rustls/rustls-ffi/target/include" \
    ./configure --prefix=$HOME/pq --with-ssl=rustls --enable-tap-tests \
        --enable-debug && \
    gmake && \
    PATH=/bin:$PATH gmake install

On subsequent calls you don't need to invoke configure, you only need to rerun:

```
gmake && PATH=/bin:$PATH gmake install
```

7. Connect to the local Postgres database using your new psql:

~/pq/bin/psql 'sslmode=verify-full sslrootcert=/Users/kevin/src/github.com/path/to/roots/root.pem host=cert_host.example.com ssl_min_protocol_version=TLSv1.2 dbname=postgres'

Running SSL tests

You need to install additional Perl modules to run the TLS tests.