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
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master

Details
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http://git.postgresql.org/pg/commitdiff/258ee1b635e43a37e901fd5f62bdd5f1087d65a5

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