pgsql: Remove the datetime keywords ABSTIME and RELTIME, which we'd been

Log Message:
-----------
Remove the datetime keywords ABSTIME and RELTIME, which we'd been treating as
noise words for the last twelve years, for compatibility with Berkeley-era
output formatting of the special INVALID values for those datatypes.
Considering that the datatypes themselves have been deprecated for awhile,
this is taking backwards compatibility a little far. Per gripe from Josh
Berkus.

Modified Files:
--------------
pgsql/doc/src/sgml:
datetime.sgml (r2.59 -> r2.60)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datetime.sgml?r1=2.59&r2=2.60)
pgsql/src/backend/utils/adt:
datetime.c (r1.202 -> r1.203)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.202&r2=1.203)
nabstime.c (r1.159 -> r1.160)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/nabstime.c?r1=1.159&r2=1.160)
pgsql/src/interfaces/ecpg/pgtypeslib:
dt_common.c (r1.47 -> r1.48)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/dt_common.c?r1=1.47&r2=1.48)
pgsql/src/test/regress/expected:
timestamp.out (r1.39 -> r1.40)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/timestamp.out?r1=1.39&r2=1.40)
timestamptz.out (r1.33 -> r1.34)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/timestamptz.out?r1=1.33&r2=1.34)
pgsql/src/test/regress/sql:
timestamp.sql (r1.22 -> r1.23)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/timestamp.sql?r1=1.22&r2=1.23)
timestamptz.sql (r1.19 -> r1.20)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/timestamptz.sql?r1=1.19&r2=1.20)