load_plugin: remove a check that the binary exists when a plugin command runs - the registration was successful, so the binary should exist. The check was remove to allow the -p option for load_plugin to work.

Add a -p option to load_plugin, given this option, the command does not require an absolute path for theplugin. Instead, the shell searches PATH for it.

1. Replace system() with a function that responds to SIGINT. 2. Add functions to cleanup temporary files on SIGINT. 3. Fix bugs related to signal handling.