Verify validity of sudoers file when reconfiguring system.

[pkill9]

Last time I tested, the sudoers file could be changed to anything in
the guix system configuration, whether it's valid or not. This could
result in someone being locked out of their system when root doesn't
have a password, and they rely on sudo. Ideally, `guix system
reconfigure` would fail if the specified sudoers file is invalid.

I ran `visudo --help` and there are two flags that could be used for
this: --check, which simply parses the sudoers file and checks that
it's valid, and --file, which specifies which file to check.