cpu_sched.cpp

    in particular, they were preempting jobs in the middle of time slice.
    Solution:
    1) don't use MT in the sort order defined by more_important().
    2) add a 2nd reordering in which MT jobs are moved ahead
        of non-MT jobs, but only if #CPUs used is < #CPUs
        (see promote_multi_thread_jobs())

svn path=/trunk/boinc/; revision=19312