* mount: keep the xattr flag constants off freebsd
x/sys/unix has no XATTR_CREATE or XATTR_REPLACE there, and weedfs_xattr.go
is already tagged away from freebsd for that reason. Putting them in a
!windows file dragged them back in, so master stopped building for
freebsd.
* ci: cross-compile freebsd and darwin too
The windows-only check missed a freebsd break in the very file it was
added to guard, because nothing else on a pull request compiles them.