Implement SSPI authentication

[eltoder]

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 #142

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

[eltoder]

@elprans I updated documentation and renamed the extra to hopefully make it more clear. Please take another look.

[eltoder]

@elprans when are you planning to make the next release?

[eltoder]

@elprans friendly ping