Add support for Unix sockets on Windows

[enocom]

Windows has added support for Unix sockets. Postgres also includes support for Unix sockets on Windows:

On Unix, an absolute path name begins with a slash. On Windows, paths starting with drive letters are also recognized.

Right now, pgx (through pgconn) does not support using a Unix socket on Windows and instead assumes all Unix sockets begin with a forward slash. If pgx were to change this assumption, pgx would support Unix sockets on Windows. Using a forked copy of pgconn, I've verified this change works on Windows Server 2019.

Here are the two places that would need to change:

1. https://github.com/jackc/pgconn/blob/beb4e2cfbcd7f41e6389254496cc5feae7d99d9c/pgconn.go#L190
2. https://github.com/jackc/pgconn/blob/84e8238fa074e88b15e2737f4d8a0a8abb1ae6bf/config.go#L106

Rather than:

if strings.HasPrefix(fb.Host, "/") {
  // ...
}

The code could be:

if strings.HasPrefix(fb.Host, "/") || isWindowsPath(fb.Host) {
  // ...
}

where isWindowsPath checks if the provided string begins with a capital letter A-Z followed by a colon and a backslash (e.g., C:\).

Would there be any interest in accepting this feature request?

[jackc]

Sounds good to me.

[enocom]

I'll send a PR.