Enhance callgrind option --collect-system.

Enhance callgrind option --collect-system.

  - The command option --collect-systime has been enhanced to specify
    the unit used to record the elapsed time spent during system calls.
    The command option now accepts the values no|yes|msec|usec|nsec,
    where yes is a synonym of msec.  When giving the value nsec, the
    system cpu time of system calls is also recorded.

Note that the nsec option is not supported on Darwin.