`sin_port` should be in network byte order.

Introduce a simple counter that `p_fsync` implements.  This is useful
for ensuring that `p_fsync` is called when we expect it to be, for
example when we have enabled an odb backend to perform `fsync`s when
writing objects.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

While often similar, these are not the same on Windows. We want to use the page
size on Windows for the pools, but for mmap we need to use the allocation
granularity as the alignment.

On the other platforms these values remain the same.

Ensure that the given length to `p_read` is of ssize_t and ensure
that callers test the return as if it were an `ssize_t`.

Use size_t for page size, instead of long. Check result of sysconf.
Use size_t for page offset so no cast to size_t (second arg to p_mmap).
Use mod instead div/mult pair, so no cast to size_t is necessary.