parser: Add the ability to turn off jobs to ease with debugging

The parser currently uses a fork model to do job processing. For consistency even when the number of jobs is set to 1 a single work process is forked. However this makes using gdb more difficult and can be even worse for other debugging tools.

Make -j 0 disable all job spawning so all processing happens in the main process.

Signed-off-by: John Johansen john.johansen@canonical.com