[nemo] Implement checking of effective GID of caller

This allows us to do fine-grained permission checking without
the limitations of the D-Bus busconfig XML configuration files.

Caveats right now:

  - Malicious apps can listen to the passwordChanged signal
    for generated passwords, and get the generated password
  - Malicious apps can listen to calls and capture a manually-
    set password by monitoring the setPassword call
[nemo] Don't send updated password as D-Bus signal

This makes sure no unprivileged process can monitor the signal on the
system bus while a privileged app updates the password.
Merge pull request #5 from thp/caller-is-setgid-privileged

[nemo] Implement checking of effective GID of caller