Closes #497

Closes #485

This can happen with images that have ridiculous dimensions.

This ensures that X11 layout or other changes do not affect the .fehbg
contents, which are meant to only be updated when a --bg-option is used
manually.

Closes #482

When setting wallpapers without --no-fehbg option, a ~/.fehbg file is
created. This file is set to be an executable for later re-use.

Calling stat() without checking the return value can lead to issues.
If the call fails, then s.st_mode is undefined and excessive permissions
could be set to .fehbg, at worst even setuid/setgid bits for a world
writable file.

While adjusting this, I changed the code to use fstat() and fchmod() to
avoid a further -- but very unlikely -- issue: race condition in form of
TOCTOU. If the file ~/.fehsetbg is replaced by a symlink right before
the chmod call, then a different file would be set executable + the
default mode of the (newly created) file. I don't expect this to be a
real world issue but changed this part "while at it" anyway for more
robust code and a good example on how to handle files.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

That part slipped through while reviewing the PR...

Update "examples/find-lowres"

Closes #463

Closes #456

Otherwise, several --info switches will not properly override each other

This fixes "feh -D -1 -D 1" and similar starting the slideshow in paused mode
even though the last -D option specifies autostart.

Closes #456

Note that --randomize and directory names are not saved in fehbg. It only
contains the command line needed to recreate the wallpaper setup which
resulted from its invocation, i.e., after randomization etc. have been
applied. This is intentional.