pgsql: Provide a way to supress the "out of memory" error when allocati

Provide a way to supress the "out of memory" error when allocating.

Using the new interface MemoryContextAllocExtended, callers can
specify MCXT_ALLOC_NO_OOM if they are prepared to handle a NULL
return value.

Michael Paquier, reviewed and somewhat revised by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/bd4e2fd97d3db84bd970d6051f775b7ff2af0e9d

Modified Files
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src/backend/utils/mmgr/mcxt.c |   40 ++++++++++++++++++++++++++++++++++++++++
src/include/utils/palloc.h    |    9 +++++++++
2 files changed, 49 insertions(+)