pwait: Add an option to print remaining processes

• On startup, insert all valid PIDs into a tree.
• In our main loop, whenever a process terminates, remove its PID from the tree.
• On exit, if the -p flag was specified, print the remaining PIDs.

MFC after: 3 days
Reviewed by: bcr, markj
Differential Revision: https://reviews.freebsd.org/D53293