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
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/07cef3eb8a621690ff5e1a1d576656284957caf1

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