Add gnome-keyring support to NM_0_8 branch

Backported the following commits:

72f391a24487ab442290c47e2e670f52c41b0608 Use gnome-keyring for password fields
db47228f8e73b41549f89d941d52ae8e3acafb62 gnome-keyring: use the vpn_uuid instead of the host
d9f4b023fd4211affdd5a1b69305a4de78ac60f3 Only store passwords in keyring upon success
31a0e8b8cf78206cda34ab593ba90c3d28ebbc29 Handle string == NULL in gnome-keyring callback
a37b1f725c460b5237ed6ab36a961c2e3f1c8145 Remove passwords from gnome-keyring when user disables 'save passwords'
96a0741b154dbb3a629038b34c89469b9bd11adb Fix nm_process_auth_form() to clear 'form_grabbed' and cancel keyring ops
eee2f4fd0f1aa3e2439c9ca337b00f5a382d9b8e When entry with focus gets password from gnome-keyring, select the full region