netsniff-ng: ctx: init and destruct methods for ctx

Refactor ctx initialization and destruction into separate handlers.
That is more clean.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
dev: linktype: use ARPHRD_ETHER for "any" device

Use ARPHRD_ETHER for the "any" device, so that when typing "netsniff-ng"
without any arguments, ethernet is assumed.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>