debugger: pass on v8 debug switches

Before this commit, passing --debugger and other V8 debug switches to
node.js made node print a usage message and exit.

Rewrite the debug argument parser so it only consumes switches that we
understand and pass everything else as-is to V8.

A side effect of this change is that switches like --debugger_agent and
--debugger_port now work.  That kind of obsoletes our debugger switches
because they implement pretty much the same functionality but let's
leave them in for now for the sake of convenience and backwards
compatibility.

Fixes #6526.