Start to address the race issue between restoring a file's contents

Start to address the race issue between restoring a file's contents
and restoring the metadata.  In particular, the metadata-restore
functions now all accept a file descriptor and a pathname.  If the
file descriptor is set and the platform supports the appropriate
syscall, restore the metadata through the file descriptor.  Otherwise,
restore it through the pathname.  This is complicated by varying
syscall support (FreeBSD has an fchmod(2) but no fchflags(2), for
example) and because non-file entries don't have an fd to use in
restoring attributes (for example, mknod(2) doesn't return a file
handle).

MFC after: 14 days