Improved Gentoo ebuild querying output.

Specifically, print_ebuild() now outputs the passed ebuild's direct
dependencies, installed files, latest changes, metadata, and use flags.
Added reasonably intelligent add_user:() and remove_user:() functions.
Fixed the long-standing standard output mtime issue in @{io/output/output}.
While less intelligent, standard output state trackers (e.g., start_stdout:())
no longer bother attempting to maintain mtime for standard output (and hence at
least behave reliably).