Add support for restarting when attaching with --gdb

The current implementation has two issues. The main one is that running gdb again will go back to a python interpreter, losing all the breakpoints and other settings. With this change, it is possible to restart as many times as desired. The second issue is that currently 5 seconds are spent sleeping, so it is not very fast.

To test run

gaudirun.py GaudiTestSuite/options/ControlFlow/AlgSequencer.py --gdb

Previously, you can type c and it will sleep 5 seconds and then run from where it stopped. If setting a breakpoint (like b Prescaler.cpp:22), after stopping typing r will give you a python prompt. With this change, running c will continue after the configuratio phase, and running r will start from the beginning. If a breakpoint is set, running r again will start from the beginning again, with the previous breakpoints and other settings.