Ticket #3339: fix pasting from clipboard.

When clipboard_paste option was set but clipboard utility was not
installed, nothing was pasted and clipboard file became empty.

Solution: reimplement pasting way using mc_pipe_t instead of my_system().

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Clarify flags and mode to open clipboard file.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Clarify description of clipboard_store and clipboard_paste options.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>