Adding lecture21
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Adding lecture22
Adding has_soln.c
Adding vector.c
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Adding fork_soln.c
Adding lecture23
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Adding semaphore code
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems