Add a new GUC variable called "IntervalStyle" that decouples interval output

Add a new GUC variable called "IntervalStyle" that decouples interval output
from DateStyle, and create a new interval style that produces output matching
the SQL standard (at least for interval values that fall within the standard's
restrictions).  IntervalStyle is also used to resolve the conflict between the
standard and traditional Postgres rules for interpreting negative interval
input.

Ron Mayer