wlantest: Add support for writing pcapng files

wlantest: Add support for writing pcapng files

The new -n<file> command line argument can be used to request wlantest
to write all read or captured frames into a pcapng file. This is similar
to the -w argument, but with pcapng allowing per-frame comments to be
embedded in the file.

Signed-hostap: Jouni Malinen <j@w1.fi>