Ensure libnl is sufficiently LGPL for our use

[dankamongmen]

I don't want any GPL infecting us. It appears that most files in libnl are LGPL, but a few might be GPL or other licenses. Sort this all out. If need be, replace libnl with libmnl or roll our own.

[dankamongmen]

According to the debian copyright file, we've got a mix of three licenses: LGPL 2.1 for the remainder, GPL2 for a few files, and some one-off Texas Instruments license for a few more.

GPL2:

src/nl-addr-add.c
src/nl-addr-list.c
src/nl-cls-add.c
src/cls/utils.c
src/cls/cgroup.c
src/cls/utils.h
src/cls/basic.c
src/nl-addr-delete.c:

TI:

include/netlink/xfrm/selector.h
include/netlink/xfrm/sa.h
include/netlink/xfrm/ae.h
include/netlink/xfrm/sp.h
include/netlink/xfrm/template.h
include/netlink/xfrm/lifetime.h
lib/xfrm/sa.c
lib/xfrm/template.c
lib/xfrm/ae.c
lib/xfrm/sp.c
lib/xfrm/selector.c
lib/xfrm/lifetime.c

I don't like that GPL2 list. iproute2 uses libmnl, and iproute2 has served me well for twenty years. Maybe we ought make that switch.

[dankamongmen]

for what it's worth, libmnl is also about 1/4 the size of libnl:

[schwarzgerat](0) $ ls -l /lib/x86_64-linux-gnu/libmnl.so.0.2.0 
-rw-r--r-- 1 root root 30712 2019-07-29 13:21 /lib/x86_64-linux-gnu/libmnl.so.0.2.0
[schwarzgerat](0) $ ls -l /lib/x86_64-linux-gnu/libnl-3.so.200.26.0 
-rw-r--r-- 1 root root 139160 2019-08-18 07:08 /lib/x86_64-linux-gnu/libnl-3.so.200.26.0
[schwarzgerat](0) $ 

[dankamongmen]

note that while https://www.infradead.org/~tgr/libnl/ only shows releases up through 2014's 3.2.25, https://github.com/thom311/libnl/releases seems quite up-to-date.