Initial lockscreen should unlock gnome keyring

Phosh will display an initial lockscreen, if it was not started from a display manager. In that case, it should also set up the PAM session as @guido.gunther explained in #phosh. With PAM, the password typed in by the user would be used to unlock gnome-keyring.

If you want phosh to show the initial lock screen you also expect it to handle e.g. unlocking gnome-keyring, ... (which we currenlty don't do but should) [...]

phosh will never open gnome-keyring itself, it would just resort to pam [...]

Related: brainstorming about how PAM could be handled with/without full disk encryption and with displaying as few password prompts as possible: pmaports#823 (comment)