libpq: Use modern socket flags, if available.

Since commit 7627b91cd5d, libpq has used FD_CLOEXEC so that sockets
wouldn't be leaked to subprograms.  With enough bad luck, a
multi-threaded program might fork in between the socket() and fcntl()
calls.  We can close that tiny gap by using SOCK_CLOEXEC instead of a
separate call.  While here, we might as well do the same for
SOCK_NONBLOCK, to save another syscall.

These flags are expected to appear in the next revision of the POSIX
standard, specifically to address this problem.  Our Unixoid targets
except macOS and AIX have had them for a long time, and macOS would
hopefully use guarded availability to roll them out, so it seems enough
to use a simple ifdef test for availability until we hear otherwise.
Windows doesn't have them, but has non-inheritable sockets by default.

Discussion: https://postgr.es/m/CA%2BhUKGKb6FsAdQWcRL35KJsftv%2B9zXqQbzwkfRf1i0J2e57%2BhQ%40mail.gmail.com

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