Get rid of postgres.c's separate parsing logic for PGDATESTYLE env.

variable, instead calling same code in variable.c that is used to parse
SET DATESTYLE.  Fix bug: although backend's startup datestyle had been
changed to ISO, 'RESET DATESTYLE' and 'SET DATESTYLE TO DEFAULT' didn't
know about it.  For consistency I have made the latter two reset to the
PGDATESTYLE-defined initial value, which may not be the same as the
compiled-in default of ISO.