Add o_sync option to file:open/2

The o_sync option adds the POSIX O_SYNC flag to the open system call on
platforms that support the flag or its equivalent, e.g.,
FILE_FLAG_WRITE_THROUGH on Windows. For platforms that don't support it,
file:open/2 returns {error, enotsup} if the o_sync option is passed in.

The semantics of O_SYNC are platform-specific. For example, not all
platforms guarantee that all file metadata are written to the disk along
with the file data when the flag is in effect. This issue is noted in the
documentation this commit adds for the o_sync option.

Add a test for o_sync. Note however that the underlying OS semantics for
O_SYNC can't be tested in any practical way, so the test assumes the OS
does the right thing with the flag when present. For manual verification,
dtruss on OS X and strace on Linux were both run against beam processes to
watch calls to open(), and file:open/2 was called in Erlang shells to open
files for writing, both with and without the o_sync option. Both the dtruss
output and the strace output showed that the O_SYNC flag was present in the
open() calls when o_sync was specified and was clear when o_sync was not
specified.