pgsql: pgbench: Refactor thread portability support.

pgbench: Refactor thread portability support.

Instead of maintaining an incomplete emulation of POSIX threads for
Windows, let's use an extremely minimalist macro-based abstraction for
now. A later patch will extend this, without the need to supply more
complicated pthread emulation code. (There may be a need for a more
serious portable thread abstraction in later projects, but this is not
it.)

Minor incidental problems fixed: it wasn't OK to use (pthread_t) 0 as a
special value, it wasn't OK to compare thread_t values with ==, and we
incorrectly assumed that pthread functions set errno.

Discussion: https://postgr.es/m/20200227180100.zyvjwzcpiokfsqm2%40alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b1d6a8f86813772b9198367a34c8ff8bff7fef9e

Modified Files
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src/bin/pgbench/pgbench.c | 126 ++++++++++++----------------------------------
1 file changed, 31 insertions(+), 95 deletions(-)