Since both netinet/ and netinet6/ call into netipsec/ and netpfil/,
the protocol specific mbuf flags are shared between them.

- Move all M_FOO definitions into a single place: netinet/in6.h, to
  avoid future  clashes.
- Resolve clash between M_DECRYPTED and M_SKIP_FIREWALL which resulted
  in a failure of operation of IPSEC and packet filters.

Thanks to Nicolas and Georgios for all the hard work on bisecting,
testing and finally finding the root of the problem.

PR:			kern/186755
PR:			kern/185876
In collaboration with:	Georgios Amanakis <gamanakis gmail.com>
In collaboration with:	Nicolas DEFFAYET <nicolas-ml deffayet.com>
Sponsored by:		Nginx, Inc.