Babel: rewrite w/o print buffer rotation
remove stale comment
update CREDITS
fix bittok2str_internal() w/o separator (GH #391)

Simplify separator string handling in bittok2str_internal(): use empty
value for the first snprintf() call and set new value after each use.
This makes the terminating null char management unnecessary, especially
that it missed the case where there was no separator and no match (it
would return the previous content of the static buffer unchanged).

This change may affect the output of tcpdump in that before it could
print either "[]" or "[none]" or a string like "[S.]" for TCP flags
value 0. Now it prints "[none]" as that is exactly the value passed to
bittok2str_nosep() in tcp_print().
PPP: reuse etheraddr_string()

Also rewrite a check in the same form as in the preceding switch case.
don't reinvent ND_TCHECK2()
Make droproot say something when successful.

I've seen people run into situations where they were using a command like this:

tcpdump -i eth0 -G 500 -w /root/%H%M%S.pcap

The first file would be created successfully but the second file would not
because their version of tcpdump was dropping privs. It was unclear to them
that this was going on and was causing confusion.

At least with this message in there it should become more evident that
privs are being altered and aid in debugging these kinds of problems.
Print priv drop msg here too.