Commit 428b1d6b introduced the use of
msync() for flushing dirty data from the kernel's file buffers.  Several
portability issues were overlooked, though:

* Not all implementations of mmap() think that nbytes == 0 means "map
the whole file".  To fix, use lseek() to find out the true length.
Fix callers of pg_flush_data to be aware that nbytes == 0 may result
in trashing the file's seek position.

* Not all implementations of mmap() will accept partial-page mmap
requests.  To fix, round down the length request to whatever sysconf()
says the page size is.  (I think this is OK from a portability standpoint,
because sysconf() is required by SUS v2, and we aren't trying to compile
this part on Windows anyway.  Buildfarm should let us know if not.)

* On 32-bit machines, the file size might exceed the available free
address space, or even exceed what will fit in size_t.  Check for
the latter explicitly to avoid passing a false request size to mmap().
If mmap fails, silently fall through to the next implementation method,
rather than bleating to the postmaster log and giving up.

* mmap'ing directories fails on some platforms, and even if it works,
msync'ing the directory is quite unlikely to help, as for that matter are
the other flush implementations.  In pre_sync_fname(), just skip flush
attempts on directories.

In passing, copy-edit the comments a bit.

Stas Kelvich and myself