ssh-agent: Don't have a timeout by default

IMHO, having a short timeout (1h) defeats the point of using
ssh-agent, which is not to have to retype passphrases all the time. Of
course, users who want timeouts can set programs.ssh.agentTimeout.

This restores the 14.04 behaviour.

(cherry picked from commit 63c14e259d4c8fc3ff706ccd620d3f794426ff1d)
Shut up a warning from udev

Issue #5260.

(cherry picked from commit 3d834a464ad63c8a4a4aa869fb1869fef1734702)