Rename vuslal.c:is_parent_dir{ => _at}()

It checks entry at given position rather than name, and also conflicts
with is_parent_dir() from utils/path.h.
Use is_parent_dir() in is_parent_dir_at()

To reduce code duplication.
Add dir_entry_t::was_selected field

It will be used as temporary field to store previous selection state.
Make visual mode selection code more extensible

This is required for the upcoming more advanced selection modes.
Remove check that should never be true

Check for list_pos being equal to zero follows increment, thus it
can be true only if it was -1 before, which shouldn't ever happen.
Extract visual.c:restore_previous_selection()

Restores selection of previous visual mode usage, if it was in current
directory of the view.
Introduce submodes of visual mode
Introduce AmendType to modes/visual.c

Types of selection amending.
Move visual mode indication string to visual.c
Add amending generic submode to visual mode

Includes three variations:
 - AT_APPEND - amend selection by selecting elements in scope.
 - AT_REMOVE - amend selection by deselecting elements in scope.
 - AT_INVERT - amend selection by inverting selection of elements in scope.

Also adds <c-g> key to switch between variations.
Add "av" shortcut to enter selection amending mode
Use leave_clearing_selection() in cmd_ctrl_c()

To reduce code duplication.
Extract modes/visual.c:change_amend_type()

Changes amend type and smartly reselects files.
Allow switching visual modes from within

av in v -> av
v in av -> v
Slightly better new comments in visual.c
Extract modes/visual.c:{accept,reject}_and_leave()

* Accepts selected region and leaves visual mode.

* Rejects selected region and leaves visual mode.
Fix selection after operations in visual mode

After using selection editing.
Document selection amending mode
Merge branch 'add-multiblock-selection'

Add multiblock "selection" (in quotes because it's also block
deselection and inversion of selection for completeness).  Thanks to
ranousse, Michelle Bonk (a.k.a. unixtechie).

Add av normal mode shortcut that activates selection amending mode.

Add/update av/v/V visual mode keys to switch type of current visual mode.

Add Ctrl-G key to visual selection amending mode, which switches kind of
amending (append/remove/invert).