pgsql: Fix a bug in input processing for the "interval" type.

Log Message:
-----------
Fix a bug in input processing for the "interval" type. Previously,
"microsecond" and "millisecond" units were not considered valid input
by themselves, which caused inputs like "1 millisecond" to be rejected
erroneously.

Update the docs, add regression tests, and backport to 8.2 and 8.1

Modified Files:
--------------
    pgsql/doc/src/sgml:
        datatype.sgml (r1.201 -> r1.202)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.201&r2=1.202)
    pgsql/src/backend/utils/adt:
        datetime.c (r1.179 -> r1.180)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c.diff?r1=1.179&r2=1.180)
    pgsql/src/include/utils:
        datetime.h (r1.65 -> r1.66)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/datetime.h.diff?r1=1.65&r2=1.66)
    pgsql/src/test/regress/expected:
        interval.out (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/interval.out.diff?r1=1.18&r2=1.19)
    pgsql/src/test/regress/sql:
        interval.sql (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/interval.sql.diff?r1=1.11&r2=1.12)