Add ui.c:ui_view_displays_columns() function

Checks whether view displays column view.
Add empty lines to opt_handlers.h

For better readability.
Better comment for load_view_columns_option()

Describe why it doesn't update option value.
Assert for correct option name in set_option()

Mistyping it is programmer's mistake.
Better formatting in engine/options.h

* Fix enum and union typedef declaration style.
* Add empty lines for better readability.
Several readability changes in opt_handlers.c
Restore 'viewcolumns' value on format error

E.g. now
:set viewcolumns=
:set viewcolumns=sdklfjskldfjj
:set viewcolumns?
prints "viewcolumns="
Extract opt_handlers.c:set_view_columns_option()

Updates view columns value as if 'viewcolumns' option has been changed.
Doesn't change actual value of the option, which is important for
setting sorting order via sort dialog.
Do not mess ls-like vie on setting 'viewcolumns'
Extract ui.c:ui_view_entry_target_type()

Gets real type of file view entry.
Fix dirs symlinks labels with lsview & classify

Width of a link title was calculated incorrectly as it was assumed
to be of link type rather than of directory type.
Document 'lsview' and 'viewcolumns' fixes
Merge branch 'fix-viewcolumns-and-lsview-quirks'

Fix some small issues and UI glitches related to 'lsview' and
'viewcolumns' options:

* Displaying previous value of 'viewcolumns' on setting it to incorrect
format.
* Setting 'viewcolumns' when 'lsview' is on.
* Displaying symbolic links to directories in less-like view with
custom 'classify' option value.
Return number of bytes written from copy_* funcs
Fix operations with dirs that have very long names
Merge branch 'fix-very-long-directory-names'

Fix operations with directories that have very long names.

Some buffer sizes were too small for them.
Make dedicated enumeration for sorting keys
Use SK_ prefix for sorting keys
Move is_directory_entry() to filelist.c

Checks whether entry corresponds to a directory.
Implement "(" and ")" for "perms" and "type" sort

Thanks to filterfalse.