Add "gf" and "e" keys to :grep menu

For consistency and convenience.
Fix :grep menu navigation

Broken when "gf" and "e" keys were introduced.
Make goto_selected_file() more readable
Break goto_selected_file() into functions

And make it MUCH more readable and maintainable.
Generalize code in goto_selected_file() a bit
Fix navigation to non-readable dirs from a menu

Don't check for readability when navigating, rather check for
existence.
Display value of local filter in output of :filt?
Remove GNU/screen from generic variable name

The parameter refers to both GNU/screen and TMUX.
Rename { auto_redraws -> skip_shellout_redraw }

New name shouldn't lead to that much confusion as the old one.
Merge branch 'fix-grep-navigation'

 * Add "gf" and "e" keys to :grep menu.
 * Restore navigation in :grep menu.
 * Fix navigation to non-readable directory.
 * Fix missing error message on failed file open try from a menu.
 * Display value of local filter in the output of :filter? command.
 * Refactor menu item navigation function.