Make "-lavdopts threads=0" mean an autodetected number of threads, and
make that the default value of the option. Also increase the upper
limit of the option from 8 to 16. Add new file osdep/numcores.c which
tries to determine the number of cores available on the machine.
numcores.c is based (heavily modified) on public domain numcpus.c by
Philip Willoughby <pgw99@doc.ic.ac.uk>, downloaded from
http://csgsoft.doc.ic.ac.uk/numcpus/
