Spell the values of libpq's gssdelegation parameter as "0" and "1".

Spell the values of libpq's gssdelegation parameter as "0" and "1".

That's how other boolean options are handled, so do likewise.
The previous coding with "enable" and "disable" was seemingly
modeled on gssencmode, but that's a three-way flag.

While at it, add PGGSSDELEGATION to the set of environment
variables cleared by pg_regress and Utils.pm.

Abhijit Menon-Sen, per gripe from Alvaro Herrera

Discussion: https://postgr.es/m/20230522091609.nlyuu4nolhycqs2p@alvherre.pgsql