Handle wtap_dump_fdopen() as wtap_dump_open() eg call
wtap_dump_fdopen_ng() and add a dummy IDB to be able to write pcapng files.
mergecap: Can't open or create <FILENAME>: Internal error.
We might want to add a SHB comment from mergecap giving the merged filenames or something like that, Merging of pcapng files
is a different issue, idealy we should probably start using several SHB:s in that case.