pgsql: Move DTK_ISODOW DTK_DOW and DTK_DOY to be type UNITS rather than

Move DTK_ISODOW DTK_DOW and DTK_DOY to be type UNITS rather than
RESERV. RESERV is meant for tokens like "now" and having them in that
category throws errors like these when used as an input date:

stark=# SELECT 'doy'::timestamptz;
ERROR:  unexpected dtype 33 while parsing timestamptz "doy"
LINE 1: SELECT 'doy'::timestamptz;
               ^
stark=# SELECT 'dow'::timestamptz;
ERROR:  unexpected dtype 32 while parsing timestamptz "dow"
LINE 1: SELECT 'dow'::timestamptz;
               ^

Found by LLVM's Libfuzzer

Branch
------
REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f4afbe0653022b4689e0732e308810373f6d1c46

Modified Files
--------------
src/backend/utils/adt/datetime.c  |    6 +--
src/backend/utils/adt/timestamp.c |   80 ++++++++++++++++++-------------------
2 files changed, 43 insertions(+), 43 deletions(-)