Turns out that it is undefined behaviour to pass a value to isctype functions
which does not fit inside a char. Closes #312

Closes #303

like %o and %z in slideshow actions (I would like to use this to zoom in, pan,
and then use an action to crop the window to zoomed in view).

Fixed(?) Makefile document build issue for README.md

Many image collections are organized by directory, so it is nice to have
jump-to-adjacent-directory navigation.

e.g. Given the following file hierarchy:

    .
    ├── A
    │   ├── 1.jpg
    │   ├── 2.jpg
    │   └── C
    │       ├── 1.jpg
    │       ├── 2.jpg
    │       └── 3.jpg
    └── B
        ├── 1.jpg
        ├── 2.jpg
        └── 3.jpg

`feh --recursive` creates the following filelist:

    A/1.jpg <---- current_file
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

If we press [next_dir], we move the current_file pointer to:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg <-- current_file
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

Pressing [next_dir] again moves the pointer to:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg <---- current_file
    B/2.jpg
    B/3.jpg

[next_dir] now moves the pointer back to the top of the list:

    A/1.jpg <---- current_file
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

Pressing [prev_dir] from here moves backwards to the first image of the
previous directory:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg <---- current_file
    B/2.jpg
    B/3.jpg

When starting from an position that is not the first image of a
directory, [prev_dir] moves the pointer to the first image of the
current directory.

These actions combine well with `--sort dirname` since all regular files
in a directory will be sorted before any subdirectories, avoiding a
filelist like the following:

    A/1.jpg
    A/SUBDIR/2.jpg
    A/SUBDIR/3.jpg
    A/4.jpg

With `--sort dirname` that filelist becomes:

    A/1.jpg
    A/4.jpg
    A/SUBDIR/2.jpg
    A/SUBDIR/3.jpg

previously, a button/key definition with an invalid action name would assign
the specified key to the most recent valid action. E.g. "zoom_in 4\ninvalid 5"
wuold assign button 5 to zoom_in.

this actually uses the old "force geometry" workaround to get the
scale down behaviour (which makes the code a lot cleaner) by
first setting the geometry after the window is created (to avoid
creating a 0x0 sized window, which X does not like).

this introduces a bug in feh_wm_set_bg, which will be fixed in the next
commit (only applies when run without HOME)

Patch by sdaau on IRC. Thanks!

This reverts commit b257e011.

Conflicts:

	ChangeLog

This reverts commit b6a1cff6.

Only if a previously selected item (mouse over, keyboard) is available
though.

This reverts commit 8b79bc33.

This fixes a segfault in --reload when the image becomes unloadable while feh
is running.

Note that this does not affect --reload with more than one file: If the
current image is removed, feh will simply jump to the next image. feh will
only quit if the file still exists, but can no longer be loaded.

* remove unused variable Im1 in filelist.c
* options are initialised as 0, so only explicitly set those to 0 for which it
  makes sense because of their context
* Make a few warnings about wrong option combinations fatal