Add more granular options on when to open a selected message.

Add more granular options on when to open a selected message.

There are now several checkboxes in Display/Summaries page.
The checkbox in folder open selection dialog has been removed,
since that checkbox is among the new ones.

The option "always_show_message_when_selected" is now boolean,
instead of a tri-state.

This commit also removes function summary_display_by_msgnum(),
and its functionality was moved to summary_select_by_msgnum(), which
has a new boolean argument to control whether or not to display the
selected message.