PTRACE HARDENING: few cleanups.
Add a change missing in r271916.
Fix a typo.
Fix build.
Update and clarify comments.  Remove the useless counter for impossible, but
seen in wild situation (on buggy hypervisors).

In collaboration with:	bde
MFC after:	1 week
Merge remote-tracking branch 'freebsd/master' into hardened/current/master
Merge remote-tracking branch 'freebsd/master' into hardened/current/hardening
Merge remote-tracking branch 'freebsd/master' into hardened/current/segvguard
Merge remote-tracking branch 'freebsd/master' into hardened/current/ptrace
Merge remote-tracking branch 'freebsd/master' into hardened/current/log
Merge remote-tracking branch 'freebsd/master' into hardened/current/userlandenhanced
Merge branch 'hardened/current/master' into hardened/current/unstable
Merge branch 'hardened/current/hardening' into hardened/current/unstable
Merge branch 'hardened/current/segvguard' into hardened/current/unstable
Merge branch 'hardened/current/ptrace' into hardened/current/unstable
Merge branch 'hardened/current/log' into hardened/current/unstable
Merge branch 'hardened/current/userlandenhanced' into hardened/current/unstable