Add a possibility to use expect+su

[7c6f434c]

For me, PAM is happy to let a user autenticate as the same user, but not as anyone else. So to stop running Synapse as root I added support for using expect and su instead. Works for me with Synapse 0.99.0.

[14mRh4X0r]

I'd rather not merge this, for two reasons:

• Using su will use a different PAM identifier
• Authenticating people using /etc/shadow can be done by adding the matrix user to the shadow group

[7c6f434c]

Fair enough.

Of course, adding matrix user to shadow is not very attractive because it allows exfiltration for a GPU bruteforce if a mere arbitrary-read vulnerability is found in Synapse instead of going through su with timeouts on each failure.

I guess a proper solution would be setgid-shadow helper (with an additional benefit of not having to do -c true, and not using expect to fight su). Not sure I am currently motivated to implement it, though.