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

On some systsems sysconf() can return a very large value, unsuitable for
use with malloc().  Only use sysconf() if HOST_NAME_MAX isn't avalable.

FreeBSD lacks the constant HOST_NAME_MAX, instead using sysconf(3) to
find out the value of the maximum host name length at run time.  Patch
to use this instead of HOST_NAME_MAX.
This brings with it the need to use malloc instead of using a statically
sized buffer for the host name, since the size of the buffer cannot be
known at run time.  Errors from sysconf or malloc just means that the
entire block of code is skipped over (the same way it's skipped if the
call to gethostname() fails), rather than returning any kind of error to
the caller or logging an error message somewhere.

This is kinda sloppy coding (feh --filelist and --bg-* will fail when running
on a system with path lengths >= 4096 and PATH_MAX not defined), but that's
sufficiently improbable to be okay. (plus, I ain't getting paid for this, so
if you want to improve it feel free to do so)

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

Sort filelist by dirname, then by name. This results in file entries
sorting before subdirectory entries.

Useful in conjunction with upcoming prev_dir and next_dir navigation
actions.

We did not preload when SORT_MTIME, so check opt.sort > SORT_MTIME
before offering to sort by file size.

The CB_* enum block was run through s/, /,\n\t/g for legibility.

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.

The if statements are needed because we cannot pass a NULL pointer to
XFree.