Improved process ID (PID) facilities.

:set_list_to_process_pids_if_found() provides reasonably cross-platform-portable
access to the set of all PIDs corresponding to the passed command path. Also
refactored @{00-core} to eliminate unneeded hierarchy, shifting all former
subdirectories of @{00-core/20-function} directly into @{00-core} and renaming
@{00-core/20-function/00-core} to @{00-core/20-tester}. Readable, we dare say!