src/vipw.c: Use string literals to initialize 'Prog'

src/vipw.c: Use string literals to initialize 'Prog'

This avoids using argv[0], which is controlled by the user,
and might inject arbitrary text in stderr and the logs.

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>