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