Import pam.c from uxlaunch project

This code will be used for a new daemon that starts a logind session for
a user, added in forthcoming commits.
pam: add a header; remove undefined/unused functions
Add a program for launching user sessions

To integrate graphical user sessions with systemd > 204, there must be a
daemon running that opens a PAM session before the user session starts
up.

This daemon takes one argument of form SEAT-UID. It will start a logind
session for user with UID on SEAT.
build: compile and install the new service

This service can be enabled by running

  systemctl enable user-session-launch@seat0-1000.service

for example.
Update License header here.
Squash cast warning - pam wants (int) but gets a (size_t)
Update README
Removing user-session@.service, plus references.
Use setresuid to permanently drop root privs.
revving to v9 to test for release