Enhance debugserver's err reporting on attach fails

Explicitly check for a request to attach to a pid that doesn't
exist, to attach to a pid that is already being debugged, unify the
SIP process check, and an attempt at checking if developer mode is
enabled on the system (which isn't working in debugserver, for some
reason; I can't get the authorization record which should be an
unprivileged operation and works in a standalone program I wrote).

I'll debug the developer mode check later, but I wanted to land it
along with everything else; right now it will claim that developer
mode is always enabled so it's harmless to include as-is.

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