It adds a '-j N' (= 'number of jobs') option for the 'install' command, which
can be used to specify the number of concurrent workers. If possible, at most
N packages will be built concurrently.

This version of the patch is less featureful than Mikhail's version but also
rather simpler. The key difference compared to Mikhail's version is that this
version is lacking the output serialisation and the ability to tag each output
message with the task it came from. All output is interleaved. The next step
will be to make parallel builds log to files rather than the console and only
to display a summary on the console.

In addition to not having to change the output functions, the code is a bit
simpler by keep the structure of the code the same as before, rather than
splitting it into a number of concurrent tasks with channels. Instead each
task simply executes the same pattern of install actions and concurrency
limits are enforced using semaphores.