Factor some code out of nfsm_dissct() into separate functions.

Factoring some of the code in nfsm_dissct() out into separate functions
allows these functions to be used elsewhere in the NFS mbuf handling code.
Other uses of these functions will be done in future commits.
It also makes it easier to add support for ext_pgs mbufs, which is needed
for nfs-over-tls under development in base/projects/nfs-over-tls.

Although the algorithm in nfsm_dissct() is somewhat re-written by this
patch, the semantics of nfsm_dissct() should not have changed.