Add pn_alloc()/pn_free() functions

Add pn_alloc()/pn_free() functions

In order to remove the HAVE_PN_UTILS wrappers the pn_alloc() and
pn_free() functions must be implemented.  The existing illumos
implementation were used for this purpose.

The `flags` argument which was used in places wrapped by the
HAVE_PN_UTILS condition has beed added back to zfs_remove() and
zfs_link() functions.  This removes a small point of divergence
between the ZoL code and upstream.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4522