MFC r276148:
Remove in_gif.h and in6_gif.h files. They only contain function
declarations used by gif(4). Instead declare these functions in C files.
Also make some variables static.

MFC r276215:
Extern declarations in C files loses compile-time checking that
the functions' calls match their definitions. Move them to header files.

MFC r282965:
Add an ability accept encapsulated packets from different sources by one
gif(4) interface. Add new option "ignore_source" for gif(4) interface.
When it is enabled, gif's encapcheck function requires match only for
packet's destination address.

Differential Revision: https://reviews.freebsd.org/D2004
Sponsored by: Yandex LLC

MFC r273087 (with modifications):
Overhaul if_gif(4):
o convert to if_transmit;
o use rmlock to protect access to gif_softc;
o use sx lock to protect from concurrent ioctls;
o remove a lot of unneeded and duplicated code;
o remove cached route support (it won't work with concurrent io);
o style fixes.

MFC r273090:
Move memset under ifdef INET6.

MFC r273091:
Add more ifdefs. SIOC*_IN6 are defined only with INET6.

MFC r273121:
Add inet/inet6 to the dependency list. Without them if_gif is useless.

MFC r273209 by bz:
After r273087,r273090,r273091,r273121 changes to gif(4) try to fix
NOIP builds for real.

MFC r273587:
Remove redundant check and m_pullup() call.

MFC r269054:

Fix EtherIP. TOS field must be initialized when the inner protocol is
PF_LINK, and multicast/broadcast flag should always be dropped because
the outer protocol uses unicast even when the inner address is not for
unicast. It had been broken since r236951 when gif_output() started to
use IFQ_HANDOFF().

MFC r276148:
Remove in_gif.h and in6_gif.h files. They only contain function
declarations used by gif(4). Instead declare these functions in C files.
Also make some variables static.

MFC r276215:
Extern declarations in C files loses compile-time checking that
the functions' calls match their definitions. Move them to header files.

MFC r282965:
Add an ability accept encapsulated packets from different sources by one
gif(4) interface. Add new option "ignore_source" for gif(4) interface.
When it is enabled, gif's encapcheck function requires match only for
packet's destination address.

Differential Revision: https://reviews.freebsd.org/D2004
Sponsored by: Yandex LLC

MFC r273087 (with modifications):
Overhaul if_gif(4):
o convert to if_transmit;
o use rmlock to protect access to gif_softc;
o use sx lock to protect from concurrent ioctls;
o remove a lot of unneeded and duplicated code;
o remove cached route support (it won't work with concurrent io);
o style fixes.

MFC r273090:
Move memset under ifdef INET6.

MFC r273091:
Add more ifdefs. SIOC*_IN6 are defined only with INET6.

MFC r273121:
Add inet/inet6 to the dependency list. Without them if_gif is useless.

MFC r273209 by bz:
After r273087,r273090,r273091,r273121 changes to gif(4) try to fix
NOIP builds for real.

MFC r273587:
Remove redundant check and m_pullup() call.

MFC r269054:

Fix EtherIP. TOS field must be initialized when the inner protocol is
PF_LINK, and multicast/broadcast flag should always be dropped because
the outer protocol uses unicast even when the inner address is not for
unicast. It had been broken since r236951 when gif_output() started to
use IFQ_HANDOFF().