Implemented first draft of distro detection.

Specifically, implemented ::set_system_globals_os() in @{inspect}. Such function
has been completely specced out but incompletely implemented. Also substantially
improved @{io/stdin}, namely with a more robust :is_stdin_readable_sans_blocking()
and even more robust alternative to such tester :is_stdin_pipe_or_file_simple().
Globally renamed "file_regular" to "file_simple".