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

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.