pgsql: Fix to_char(interval) to return proper year and century values.

[momjian(at)svr1(dot)postgresql(dot)org (Bruce Momjian)]

Log Message:
-----------
Fix to_char(interval) to return proper year and century values.
Fix to_char(interval) to return large year/month/day/hour values that
are larger than possible timestamp values.
Prevent to_char(interval) format specifications that make no sense, like
Month.
Clean up formatting.c code to more logically handle return lengths.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        formatting.c (r1.97 -> r1.98)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/formatting.c.diff?r1=1.97&r2=1.98)