rename alter constraint enforceability related functions

rename alter constraint enforceability related functions

The functions AlterConstrEnforceabilityRecurse and
ATExecAlterConstrEnforceability are being renamed to
AlterFKConstrEnforceabilityRecurse and ATExecAlterFKConstrEnforceability,
respectively.

The current alter constraint functions only handle Foreign Key constraints.
Renaming them to be more explicit about the constraint type is necessary;
otherwise, it will cause confusion when we later introduce the ability to alter
the enforceability of other constraints.

Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Amul Sul <sulamul@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>

Discussion: https://postgr.es/m/CACJufxHCh_FU-FsEwsCvg9mN6-5tzR6H9ntn+0KUgTCaerDOmg@mail.gmail.com