pgsql: Make it easier to choose the used waiting primitive in unix_latc

Make it easier to choose the used waiting primitive in unix_latch.c.

This allows for easier testing of the different primitives; in
preparation for adding a new primitive.

Discussion: 20160114143931.GG10941@awork2.anarazel.de
Reviewed-By: Robert Haas

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c17966201c7de2a4c437bed6d83c6a7f2e7108f4

Modified Files
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src/backend/port/unix_latch.c | 50 +++++++++++++++++++++++++++++--------------
1 file changed, 34 insertions(+), 16 deletions(-)