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
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