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

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_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/530d0c504fe636b0ed4a3817b1e9806dd55a52b6

Modified Files
--------------
src/backend/utils/adt/datetime.c | 6 +--
src/backend/utils/adt/timestamp.c | 79 +++++++++++++++++++------------------
2 files changed, 43 insertions(+), 42 deletions(-)