Additions to support the erlide debugger

There are mostly new functions and messages that don't affect the
regular debugger. The only exception is in dbg_ieval, where the exit_at
message has more detailed information (the pid of the process, a
stacktrace and the current bindings).

The new API is about
* getting the current stacktrace
* setting the value of a variable
* storing stacktrace and bindings for exited processes, it is useful for post-mortem analysis

If these changes are accepted, erlide can stop using a copy of the debugger code with patches. Even the regular debugger might like to use these.