Implemented -e trace=%clock option

[Eugene Syromyatnikov]

> diff --git a/strace.1 b/strace.1
> index 9b69ec22..c46ca3ba 100644
> --- a/strace.1
> +++ b/strace.1
> @@ -429,6 +429,9 @@ Trace all memory mapping related system calls.
>  .BR "\-e\ trace" = %sched
>  Trace all scheduler-related (sched_*) system calls.
>  .TP
> +.BR "\-e\ trace" = %clock
> ++Trace all clock-related (clock_*) system calls.
Not sure that this plus sign in the beginning of the line is intended.