vfs: link() system call

TODO is:

 - wire up the vnode hash again, this time with a uin64_t as index
 - use the znode id as index for zfs, a counter for ramfs/devfs
 - write the actual link syscall and vnop (should be trivial)
 - lots of testing
tst-fs-link.so: link() test

Add a basic test case for filesystem hard link support. You can run it as
follows:

  ./scripts/run.py -e "tests/tst-fs-link.so"

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>