waitpid: refactor to improve maintainability

waitpid: refactor to improve maintainability

This refactor introduces 'struct waitpid_control' which is
used to control the program state, e.g. option flags, to
create context for internal routines that can adapt their
execution flow accordingly. It has as goal to make the code
easier to understand and reduces the scope of variables.

It also adds 'struct process_info' which stores information
on a process, specified by the PID passed on the command line,
such as the PID, process file descriptor and pidfd inode number.

An array of process_info is used to store information on all
specified PIDs on the command line and is used to create the
event poll list that is integral to the waitpid(1) logic.

This refactor lays the ground work to introduce support for
the 'PID:inode' process addressing format.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>