Refactor the options handling in defaults.py to allow the --jobs
option have varing specific parameters. The option supports 'none',
'max' and 'half' or a fraction to divide the number of CPUs or
an integer value which is the number of jobs. The --no-smp has
been removed.
The host specific modules have been changed to set the number of
CPUs in the defaults table.
Fixed the --keep-going to clean up is --always-clean is provided
even if the build has an error.
The git module allows basic access to git. Hosts are now required
to provide git support.
The defaults module now returns options as a list split on '='.
