Move td_sigqueue to the end of struct thread

[badger]

Summary

In order to preserve KBI in stable branches, replace the existing
td_sigqueue slot with padding and move the expanded (as of r315949)
td_sigqueue to the end of the struct.

Test Plan

I checked the offset of td_lend_user_pri (the first member after
the original td_sigqueue) in struct proc on a couple of architectures and
ensured that it was the same as it was before r315949.

[kib]

I think, it might be useful to add _Static_assert() on stable branches that e.g. offsetof(struct thread, td_pflags) is equal to the specific hard-coded values for i386, amd64 and arms. Then such issues can be catched automatically.

IMO the asserts would be useful even for head, but then some explanation in the comment might be warranted.

[jhb]

Also, to be clear, this would just be a direct commit to stable/{10,11}, it would not go into HEAD.

[badger]

In D10515#217891, @jhb wrote:

Also, to be clear, this would just be a direct commit to stable/{10,11}, it would not go into HEAD.

Understood, and thank you.