Add RTF_*, RTA_*, RTAX_*, RTM_* definitions on BSDs

[tesuji]

Grep from:

• https://github.com/freebsd/freebsd-src/raw/main/sys/net/route.h
• https://github.com/DragonFlyBSD/DragonFlyBSD/raw/master/sys/net/route.h
• https://github.com/NetBSD/src/raw/trunk/sys/net/route.h
• https://github.com/openbsd/src/raw/master/sys/net/route.h

Closes #3711.

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[rustbot]

Some changes occurred in OpenBSD module

cc @semarie

Some changes occurred in OpenBSD module

cc @semarie

[tesuji]

shall I remove this place holder?

[jbeich]

Would be inconsistent with macOS:

libc/src/unix/bsd/apple/mod.rs

Line 4770 in acc7bb1

pub const RTM_VERSION: ::c_int = 5;

Besides, RTM_VERSION is used by iroh-net.

[jbeich]

Do you plan to unify with macOS (same ancestor) by moving RTM_ADD and other constants with same values to src/unix/bsd/mod.rs ?

[jbeich]

Please, don't remove *_MAX unless applied to macOS as well:

error[E0425]: cannot find value `RTAX_MAX` in crate `libc`
   --> cargo-crates/iroh-net-0.16.2/src/net/interfaces/bsd.rs:875:46
    |
875 |     let mut addrs = Vec::with_capacity(libc::RTAX_MAX as usize);
    |                                              ^^^^^^^^ not found in `libc`

error[E0425]: cannot find value `RTAX_MAX` in crate `libc`
   --> cargo-crates/iroh-net-0.16.2/src/net/interfaces/bsd.rs:879:23
    |
879 |     for i in 0..libc::RTAX_MAX as usize {
    |                       ^^^^^^^^ not found in `libc`
    |
help: you might have meant to write `.` instead of `..`
    |
879 -     for i in 0..libc::RTAX_MAX as usize {
879 +     for i in 0.libc::RTAX_MAX as usize {
    |