fix bittok2str_internal() w/o separator (GH #391)

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().