Changing gssencmode default in Psycopg

Hello,

several Psycopg users report problems with GSS connection attempts,
which cause regular crashes on Windows and on macOS in multiprocessing
environments. Last report in [1] but looking for `gssencmode` in the
bug tracker will bring up other issues whose solution is to add a
`gssencmode=disable` to the connection string. Similar
errors/solutions can be googled for in other connector libraries
(Perl, Ruby).

Psycopg can use the system libpq, but most people rely on the `binary`
package to install a libpq in their system and use it without the need
of a C compiler on their client. I am considering changing the
parameter default to `disable` [2] in the libpq that we ship in the
macOS binary packages. I would do the same for Windows but, if I
recall correctly, at the moment we don't build a libpq.dll but we use
a pre-compiled one. Linux doesn't seem to require such treatment.

Would there be any shortcoming in doing so, apart from obviously
requiring GSS users to specify prefer/require for the parameter?

Thank you very much

-- Daniele

[1] https://github.com/psycopg/psycopg/issues/1136
[2] https://github.com/postgres/postgres/blob/master/src/interfaces/libpq/fe-connect.c#L136-L141