pgsql: Back-patch addition of the ALLOCSET_FOO_SIZES macros.

Back-patch addition of the ALLOCSET_FOO_SIZES macros.

These macros were originally added in commit ea268cdc9, and back-patched
into 9.6 before 9.6.0.  However, some extensions would like to use them
in older branches, and there seems no harm in providing them.  So add
them to all supported branches.  Per suggestions from Christoph Berg and
Andres Freund.

Discussion: https://postgr.es/m/20181012170355.bhxi273skjt6sag4@alap3.anarazel.de

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1245561df4902a08a672bd451f5f2014e03c18a9

Modified Files
--------------
src/include/utils/memutils.h | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)