linux: add sysctl to pass untranslated interface names

[melifaro]

Summary

Currently, we convert all interfaces of type IFT_ETHER to eth%d format. This was done back in the days where all Linux network interfaces were named eth%d and lots of programs assumed the existence of eth0.

Nowadays, with a plethora of virtual interfaces and Predictable Network Interface Name widely used, we can consider using unmodified interface names.

The driving factor for this diff is the upcoming support for AF_NETLINK, where real interface names are highly desired.

Test Plan

15:39 [0] m@devel0 cat /compat/linux/proc/net/dev
Inter-|                                                   Receive|  Transmit
  face|bytes    packets errs drop fifo frame compressed multicast|  bytes    packets errs drop fifo colls carrier compressed
  eth0:   39068     388    0    0    0     0          0        43   231423     388    0    0    0     0       0          0
   lo0:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0

15:52 [0] m@devel2 ip route get 1.1.1.1
0.0.0.0/0 via 10.0.0.1 dev eth0 proto unspec

AFTER:

15:52 [0] m@devel2 s sysctl compat.linux.use_real_ifnames=1
compat.linux.use_real_ifnames: 0 -> 1
15:39 [0] m@devel0 cat /compat/linux/proc/net/dev
Inter-|                                                   Receive|  Transmit
  face|bytes    packets errs drop fifo frame compressed multicast|  bytes    packets errs drop fifo colls carrier compressed
vtnet0:   52902     527    0    0    0     0          0        55   245367     484    0    0    0     0       0          0
   lo0:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0
15:51 [0] m@devel2 ip route get 1.1.1.1
0.0.0.0/0 via 10.0.0.1 dev vtnet0 proto unspec

[melifaro]

remove if_dl.h

[kib]

sys/compat/linux/linux_ioctl.c

2124

If you swap then/else bodies, you can avoid using the negation in the condition.

sys/compat/linux/linux_util.c

92

Maybe be more descriptive?
Use FreeBSD interface names instead of generating ethN aliases

331

Inner expressions do not need () around them.

sys/compat/linux/linux_util.h

126

ifp should be put into parenthesis inside the macro body