open(2): Add O_DSYNC flag.

POSIX O_DSYNC means that writes include an implicit fdatasync(2), just
as O_SYNC implies fsync(2).

VOP_WRITE() functions that understand the new IO_DATASYNC flag can act
accordingly, but we'll still pass down IO_SYNC so that file systems that
don't understand it will continue to provide the stronger O_SYNC
behaviour.

Flag also applies to fcntl(2).

Reviewed by: kib, delphij
Differential Revision: https://reviews.freebsd.org/D25090