Revise waitpid API

[Susurrus]

There are 4 different types of calls to waitpid(), all of which use a different range of a c_int to be expressed. Instead an enum type should be used that includes:

• wait for any child process with a specific process group ID
• wait for any child process
• wait for any child process whose process group ID is equal to that of the calling process
• wait for the child with a specific process ID