pgsql: Rename session_auth_is_superuser to current_role_is_superuser.

Rename session_auth_is_superuser to current_role_is_superuser.

This variable might've been accurately named when it was added in
ea886339b8, but the name hasn't been accurate since at least the
introduction of SET ROLE in e5d6b91220.  The corresponding
documentation was fixed in eedb068c0a.  This commit renames the
variable accordingly.

Suggested-by: Joseph Koshakow
Discussion: https://postgr.es/m/CAAvxfHc-HHzONQ2oXdvhFF9ayRnidPwK%2BfVBhRzaBWYYLVQL-g%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0fef8775382886bef023aee67cb744711ed7a32f

Modified Files
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src/backend/access/transam/parallel.c | 2 +-
src/backend/utils/misc/guc_tables.c   | 9 ++++++---
src/include/utils/guc.h               | 2 +-
3 files changed, 8 insertions(+), 5 deletions(-)