Minor QoL improvements

[nelsonaloysio]

A few minor quality of life tweaks I made after forking:

• Added executable privilege to the main file, i.e. chmod +x envycontrol.py
• Added --reset parameter, just executes _cleanup() and exits
• Changed --reset_sddm to --reset-sddm, as per convention
• Updated default interpreter to python3, as some system environments don't have a python executable link

Feel free to merge or cherry-pick at will. And thanks for the useful script! :-)