move p_sigqueue to the end of struct proc

[badger]

Summary

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

This is a repeat of r317529 (which concerned td_sigqueue in struct
thread) for p_sigqueue in struct proc.

Virtualbox modules are affected without this fix.

Test Plan

Check that various offsets in struct proc now match what they were in
11.0-RELEASE. Load vboxdrv kmod and start a virtual machine.