Get rid of imp, started giving deprecation warning every time in Python 3.7 in Ubuntu 19.04.

Please python stop torturing me with refactors.

Make ./run -u blow up if executable not found, otherwise I go crazy.

Get ./test-gdb back to life after the ./run relative path refactor, forgot to test this.

run-gdb-user

@@ -1,11 +1,10 @@
 #!/usr/bin/env python3
 
-import imp
 import os
 import sys
 
 import common
-rungdb = imp.load_source('run_gdb', os.path.join(kwargs['root_dir'], 'run-gdb'))
+rungdb = common.import_path_relative_root('run-gdb')
 
 parser = self.get_argparse(argparse_args={
     'description': '''GDB step debug guest userland processes without gdbserver.