Merge the last few variable.c configuration variables into the generic

GUC support.  It's now possible to set datestyle, timezone, and
client_encoding from postgresql.conf and per-database or per-user
settings.  Also, implement rollback of SET commands that occur in a
transaction that later fails.  Create a SET LOCAL var = value syntax
that sets the variable only for the duration of the current transaction.
All per previous discussions in pghackers.