Improved run_command_paged() implementation.

run_command_paged() now preserves the expected separation between
standard output and error, rather than muxing the former into the
latter. Substantially improved component "path/file/descriptor" by
correcting all outstanding regressions and nomenclature issues and
defining new openers (e.g., open_file_descriptor_to_stdout()). Split
"00-alias/00-variable/variable" into relevant subcomponents.