pgsql: Avoid assuming there will be only 3 states for synchronous_commi

Avoid assuming there will be only 3 states for synchronous_commit.
Also avoid hardcoding the current default state by giving it the name
"on" and replace with a meaningful name that reflects its behaviour.
Coding only, no change in behaviour.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/88f32b7ca29982f286b61a9d0dd29be4b8a01c25

Modified Files
--------------
src/backend/access/transam/xact.c   |    2 +-
src/backend/postmaster/autovacuum.c |    2 +-
src/backend/utils/misc/guc.c        |    2 +-
src/include/access/xact.h           |    9 ++++++---
src/include/replication/syncrep.h   |    2 +-
5 files changed, 10 insertions(+), 7 deletions(-)