NFS: unify TCP and UDP decoding, fix a bug

Change the UDP NFS printing format to match the current format of TCP
NFS printing (this complements commits f3051bc and 29d83db).

Update the man page to use the new format (this complements commit
2c86c75).

Restore printing of NFS packet length, which was broken in commit
68fe98a.
base struct tok on unsigned rather than signed int

This resolves a few Solaris-specific warnings emitted by
/opt/solarisstudio12.3/bin/cc like below:

warning: initializer does not fit or is out of range: 0xffffffff

(also happened for 0x80000000)
refine the previous change

This time eliminate a few instances of the code initializing struct tok
with negative values and squelch the warnings like below:

warning: initializer will be sign-extended: -1
dismiss decode_prefix.h

The only purpose of that header file was to provide two declarations
that now fit interface.h same well.
request more warnings during a Travis build
revert the latest change

./configure fails during the "BUILD_LIBPCAP=true BUILD_IPV6=true"
scenarios (both GCC and Clang):

Makefile:8411: warning: overriding commands for target `configure'
Makefile:455: warning: ignoring old commands for target `configure'
Makefile:8416: warning: overriding commands for target `stamp-h.in'
Makefile:460: warning: ignoring old commands for target `stamp-h.in'
Makefile:8421: warning: overriding commands for target `stamp-h'
Makefile:465: warning: ignoring old commands for target `stamp-h'
Makefile:8424: warning: overriding commands for target `Makefile'
Makefile:468: warning: ignoring old commands for target `Makefile'
Makefile:8427: warning: overriding commands for target `config.status'
Makefile:471: warning: ignoring old commands for target `config.status'
[...]
./config.status: line 1103: syntax error near unexpected token `done'
./config.status: line 1103: `done # for ac_tag'

I don't have the expertise to resolve it well right now, keep Travis CI
working meanwihle.
NDOize AH, BEEP and DTP decoders

Also remove some unnecessary passages.