Ticket #2966: fix view of broken manpages.

Initial commit: create a MC pipe class to capture stdout and stderr of
spawn processes.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
mcviewer: use mc_pipe_t to show command output.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
fixup! Ticket #2966: fix view of broken manpages.

NOT FINISHED
fixup! mcviewer: use mc_pipe_t to show command output.

NOT FINISHED!