nfsstat: Add an entry to output NFSPROC_APPENDWRITE count

Commit 5218d82c81f9 added a new NFSv4.1/4.2 procedure called
AppendWrite that uses a Verify to avoid a separate Getattr RPC
for the common case where the client knows the correct file
size for O_APPEND writes.

This patch modifies nfsstat so that it displays a count of
these new RPCs for the "-E -c" option.