Fix LINT-NOINET kernels

Fix LINT-NOINET kernels

- Generate opt_inet.h appropriately according to MK_INET_SUPPORT
- Only add in_gif.c to SRCS if MK_INET_SUPPORT != no

This is a surgical fix since the build infrastructure has diverged sufficiently
between stable/10 and head.

This change will likely be reverted if and when imp's changes to config.mk have
been backported to ^/stable/10 .

Reviewed by: ae (no serious objections)
Differential Revision: https://reviews.freebsd.org/D4864
MFC after: 1 week
Sponsored by: EMCC / Isilon Storage Division

git-svn-id: svn://svn.freebsd.org/base/stable/10@293778 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f