SSPI is a Windows technology for secure authentication. SSPI and GSSAPI
interoperate as clients and servers. Postgres documentation recommends
using SSPI on Windows clients and servers and GSSAPI on non-Windows
platforms[1].

Changes in this PR:

* Support AUTH_REQUIRED_SSPI server request. This is the same as
  AUTH_REQUIRED_GSS, except it allows negotiation with SSPI clients.

* Allow using SSPI on the client. Which library to use can be specified
  using the `gsslib` connection parameter.

* Use SSPI instead of GSSAPI on Windows by default. The latter requires
  installing Kerberos for Windows and is unlikely to work out of the
  box.

Closes MagicStack#142

[1] https://www.postgresql.org/docs/current/sspi-auth.html