mozilla2john.py: Add support for key4.db format

Description

The mozilla2john.py script included with john supports key3.db files but I noticed mozilla has switched to a new scheme and the new file is called key4.db (an SQLite 3.x database).

Steps to reproduce

• Install for instance Firefox and set a master password (also known as primary password) for saved passwords
• Find the key4.db file
  • macOS: ~/Library/Application Support/Firefox/Profiles/profile name/key4.db
  • Linux: ~/.mozilla/firefox/profile name/key4.db

I have not investigated if it is possible to extract a hash from this file but if it's possible it could be added to mozilla2john.py or maybe into a separate script.

Hints

This python program imports libnss and checks the master password using that lib. A dive into libnss is needed to investigate what is really happening. A first look suggest to investigate the PK11_CheckUserPassword() function inside libnss. If I find something more out I will add a comment below.

Start with run/mozilla2john.py and compare its key3.db handling with the key4.db SQLite format described in the issue. Read firefox_decrypt and investigate libnss, especially PK11_CheckUserPassword(), to determine whether a hash can be extracted. Done means key4.db support is added to mozilla2john.py or a separate script, if extraction is feasible.