In order to reduce duplication between MD parts of the Linuxulator

In order to reduce duplication between MD parts of the Linuxulator
move bits that are MI out into the headers in compat/linux.
For that remove bogus _packed attribute from struct l_sockaddr
and use MI types for struct members.

And continue to move into the linux_common module a code that is
intended for both Linuxulator modules (both instruction set - 32 & 64 bit)
or for external modules like linsysfs or linprocfs.

To avoid header pollution introduce new sys/compat/linux_common.h header.

Reviewed by:	emaste
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D20137