Refactor AlterRole()

Refactor AlterRole()

Get rid of the three-valued logic for the Boolean variables to track
whether the value was been specified and what the new value should be.
Instead, we can use the "dfoo" variables to determine whether the
value was specified and should be applied.  This was already done in
some cases, so this makes this more uniform and removes one layer of
indirection.

Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/8c1a2e37-c68d-703c-5a83-7a6077f4f997@enterprisedb.com