Add NetBSD support

Add NetBSD support

The bulk of it is just about adding ifdef __NetBSD__ where there is
already an ifdef __FreeBSD__

Add a arch=netbsd to deal with NetBSD specifics. I suggests that
arch=bsd could be renamed to arch=freebsd

NetBSD specific linking with -lperfuse

NetBSD patches to lib/mount.c. It turned to be less itrusive to patch
mount;c than mount_bsd.c. I suggest mount_bsd.c could be renamed to
mount_freebsd.c

Patch from Emmanuel Dreyfus