Improve the granularity of PQsocketPoll's timeout parameter?

In [1] Dominique Devienne complained that PQsocketPoll would be
far more useful to him if it had better-than-one-second timeout
resolution. I initially pushed back on that on the grounds that
post-beta1 is a bit late to be redefining public APIs. Which it is,
but if we don't fix it now then we'll be stuck supporting that API
indefinitely. And it's not like one-second resolution is great
for our internal usage either --- for example, I see that psql
is now doing

end_time = time(NULL) + 1;
rc = PQsocketPoll(sock, forRead, !forRead, end_time);

which claims to be waiting one second, but actually it's waiting
somewhere between 0 and 1 second. So I thought I'd look into
whether we can still change it without too much pain, and I think
we can.

The $64 question is how to represent the end_time if not as time_t.
The only alternative POSIX offers AFAIK is gettimeofday's "struct
timeval", which is painful to compute with and I don't think it's
native on Windows. What I suggest is that we use int64 microseconds
since the epoch, which is the same idea as the backend's TimestampTz
except I think we'd better use the Unix epoch not 2000-01-01.
Then converting code is just a matter of changing variable types
and adding some zeroes to constants.

The next question is how to spell "int64" in libpq-fe.h. As a
client-exposed header, the portability constraints on it are pretty
stringent, so even in 2024 I'm loath to make it depend on <stdint.h>;
and certainly depending on our internal int64 typedef won't do.
What I did in the attached is to write "long long int", which is
required to be at least 64 bits by C99. Other opinions are possible
of course.

Lastly, we need a way to get current time in this form. My first
draft of the attached patch had the callers calling gettimeofday
and doing arithmetic from that, but it seems a lot better to provide
a function that just parallels time(2).

BTW, I think this removes the need for libpq-fe.h to #include <time.h>,
but I didn't remove that because it seems likely that some callers are
indirectly relying on it to be present. Removing it wouldn't gain
very much anyway.

Thoughts?

regards, tom lane

[1] https://www.postgresql.org/message-id/CAFCRh-8hf%3D7V8UoF63aLxSkeFmXX8-1O5tRxHL61Pngb7V9rcw%40mail.gmail.com