Add support for ext_pgs mbufs to nfsrvd_readdir() and nfsrvd_readdirplus().

This patch code that optionally (based on ND_TLS, never set yet) generates
readdir replies in ext_pgs mbufs.
To trim the list back, a new function that is ext_pgs aware called
nfsm_trimtrailing() replaces newnfs_trimtrailing().
newnfs_trimtrailing() is no longer used, but will be removed in a future
commit, since its removal does modify the internal kpi between the NFS
modules.

This is another in the series of commits that add support to the NFS client
and server for building RPC messages in ext_pgs mbufs with anonymous pages.
This is useful so that the entire mbuf list does not need to be
copied before calling sosend() when NFS over TLS is enabled.

Use of ext_pgs mbufs will not be enabled until the kernel RPC is updated
to handle TLS.