Prevent build on unsupported OSes.

This limits the build to explicitly listed OSes.  This makes it easier to
import bazil.org/fuse in projects that support more OSes than bazil.org/fuse.
For an example of why this is helpful, see https://camlistore.org/r/2056

Tested with:
$ GOOS=freebsd go build -v bazil.org/fuse/...
bazil.org/fuse
bazil.org/fuse/syscallx
bazil.org/fuse/fs/fstestutil
$ GOOS=linux go build -v bazil.org/fuse/...
bazil.org/fuse
bazil.org/fuse/syscallx
bazil.org/fuse/fuseutil
bazil.org/fuse/fs
bazil.org/fuse/fs/fstestutil
bazil.org/fuse/fs/fstestutil/record
bazil.org/fuse/hellofs
$ GOOS=darwin go build -v bazil.org/fuse/...
bazil.org/fuse
bazil.org/fuse/syscallx
bazil.org/fuse/fuseutil
bazil.org/fuse/fs
bazil.org/fuse/fs/fstestutil
bazil.org/fuse/fs/fstestutil/record
bazil.org/fuse/hellofs