pgsql: Use unnamed POSIX semaphores on Cygwin.

Use unnamed POSIX semaphores on Cygwin.

Testing on CI showed that Cygwin's semctl() can fail with EAGAIN
(possibly due to resource limits in cygserver that could be tuned, not
examined).  Switch to so-called POSIX semaphores instead, which don't
seem to fail in that way (possibly due to a more direct implementation
using Windows semaphore primitives instead of talking to cygserver,
based on a cursory glance at the source).

Other known problems still prevent PostgreSQL from running on Cygwin
without random crashes, but this rarer problem was noticed while
testing.

Discussion: https://postgr.es/m/CA%2BhUKG%2BQ6DU4Ov9LrvUyDcF3oHS4KMRVSKmVGaeePq-kOyG9gA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f2857af485a00ab5dbfa2c83af9d83afe4378239

Modified Files
--------------
meson.build         | 1 +
src/template/cygwin | 5 +++++
2 files changed, 6 insertions(+)