src/: Hardcode Prog to known value

src/: Hardcode Prog to known value

Set Prog (program name) based on hardcoded value instead of argv[0].
This is to help prevent escape sequence injection.

Cherry-picked-from: e6c2e4393784 ("Hardcoding Prog to known value")
Link: <https://github.com/shadow-maint/shadow/issues/959>
Link: <https://github.com/shadow-maint/shadow/pull/960>
Cc: "Skyler Ferrante (RIT Student)" <sjf5462@rit.edu>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Karel Zak <kzak@redhat.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Christian Brauner <christian@brauner.io>
Signed-off-by: Alejandro Colomar <alx@kernel.org>