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.