Retire procfs-based process debugging.

[jhb]

Summary

Modern debuggers and process tracers use ptrace() rather than procfs
for debugging. ptrace() has a supserset of functionality available
via procfs and new debugging features are only added to ptrace().
While the two debugging services share some fields in struct proc,
they each use dedicated fields and separate code. This results in
extra complexity to support a feature that hasn't been enabled in the
default install for several years.

Test Plan

• passes make tinderbox and ptrace_test tests

[emaste]

no objection here

[jhb]

Note that there is a parent review to add a deprecation notice that is waiting for an exa-run. I will do a separate exp-run for this one after the first is committed since this removes the header entirely.

[kib]

sys/amd64/amd64/vm_machdep.c

205

Isn't TDB_STOPATFORK enough, and even better for the same purpose ?

[jhb]

I've rebased the patch set, but need to let a tinderbox build finish. After that I will refresh the review and request an exp-run.

[jhb]

• Rebase.
• Drop comment about PTRACE_FORK.
• Remove #warning workarounds.
• Remove pioctl.h.

[jhb]

exp-run requested at https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=244939