buildman: Add a way to limit the number of buildmans

buildman: Add a way to limit the number of buildmans

Buildman uses all available CPUs by default, so running more than one or
two concurrent processes is not normally useful.

However in some CI cases we want to be able to run several jobs at once
to save time. For example, in a lab situation we may want to run a test
on 20 boards at a time, since only the build step actually takes much
CPU.

Add an option which allows such a limit. When buildman starts up, it
waits until the number of running processes goes below the limit, then
claims a spot in the list. The list is maintained with a temporary file.

Note that the temp file is user-specific, since it is hard to create a
locked temporary file which can be accessed by any user. In most cases,
only one user is running jobs on a machine, so this should not matter.

Signed-off-by: Simon Glass <sjg@chromium.org>