Document and enforce assumptions about struct (in6_)ifreq.

[brooks]

Summary

• The two types must be type-punnable for shared members of ifr_ifru. This allows compatibility accessors to be shared.

• There must be no padding gap between ifr_name and ifr_ifru. This is assumed in tcpdump's use of SIOCGIFFLAGS output which attempts to be broadly portable. This is true for all current architectures, but very large (256-bit) fat-pointers could violate this invariant.

[brooks]

• Use _Static_assert which is defined in sys/cdefs.h rather than CTASSERT
• Use 0 not NULL for files with strange include orders.

[kib]

sys/net/if.h

428 ↗

(On Diff #40921)

I suggest to move the asserts to some .c file. There is no benefits of forcing all users of if.h to recheck the same stuff, one assert is enough for the developer to notice the issue.

[brooks]

• Move asserts to .c files.