posix-timers: Show clock ID in proc file

Expand information about posix-timers in /proc/<pid>/timers by adding
info about clock, with which the timer was created. I.e. in the forth
line of timer info after "notify:" line go "ClockID: <clock_id>".

Signed-off-by: Pavel Tikhomirov <snorcht@gmail.com>
Acked-by: Pavel Emelyanov <xemul@parallels.com>