handle interruped read and write calls

[Alberts Muktupāvels]

Use SA_RESTART flag to make sure that interrupted calls to read and/or write are restarted.

In my case read is interrupted when reading login1_session_id. Result of not handling EINTR error is that login1_session_id is set to NULL and console_kit_cookie gets login1 session id.

Related warnings in lightdm.log:
[+0.44s] WARNING: Session pid=617: Error reading from session: Interrupted system call
[+0.70s] DEBUG: Activating VT 7
[+0.70s] DEBUG: Activating ConsoleKit session c1
[+0.70s] WARNING: Error getting ConsoleKit session: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.ConsoleKit was not provided by any .service files

And because login1_session_id is null fix for 1637758 bug does not work.

[Launchpad Janitor (janitor)]

This bug was fixed in the package lightdm - 1.21.3-0ubuntu1

---------------
lightdm (1.21.3-0ubuntu1) zesty; urgency=medium

  * New upstream release:
    - Fix crashes introduced in 1.21.2 due to environment variable changes
    - Fix incorrect unref in XDMCP server code
    - Fix logging warning

 -- Robert Ancell <email address hidden> Fri, 09 Dec 2016 14:04:49 +1300

[Steve Langasek (vorlon)]

The Precise Pangolin has reached end of life, so this bug will not be fixed for that release