Add procctl(2) PROC_TRACE_CTL command to enable or disable debugger
attachment to the process. Note that the command is not intended to
be a security measure, rather it is an obfuscation feature,
implemented for parity with other operating systems.

Discussed with: jilles, rwatson
Man page fixes by: rwatson
Sponsored by: The FreeBSD Foundation
MFC after: 1 week