process a list of work in parallel

because multiprocess gives strange problems on occaision
support option --zzz
catch Ctrl-C, Broken pipe
when --zzz, num_proc must be 1
added comments
use parallel.do()
causes many newlines to be printed for Ctrl-C
say bye to multiprocessing
add docstring
Merge branch 'parallel'

Do away with Python's multiprocessing module; it has weird issues, like
failing to start once in a hundred runs.
So it is replaced by a forking synctool.parallel.do()
This makes it UNIX-only (but I bet there are more UNIX-only things in
synctool. synctool was meant for UNIX only anyway)