Closes #532

Closes #521

feh_file_info_load is called even if file_info is already populated, so
the original file_info struct is never freed. This results in a leak of
~44 Byte for each subsequenc image load

It's called conversion-cache for now, as it will also be used for
ImageMagick/dcraw-sourced images in the future. This should also help
avoid confusion between HTTP/ImageMagick/dcraw cache (files on disk),
Imlib2 cache (images in memory), and thumbnail cache.

Merge branch 'start-at-supports-urls' of https://github.com/iglosiggio/feh into iglosiggio-start-at-supports-urls

Building feh 3.3 on CentOS 7 x86_64 warns `curl_quit_function` in `imlib.c` is unused:

```
cc -g -O2 -Wall -Wextra -pedantic -std=c11 -D_POSIX_C_SOURCE=200809L -D_XOPEN_SOURCE=700 -DHAVE_LIBCURL -DHAVE_VERSCMP -DHAVE_LIBXINERAMA -DHAVE_LIBEXIF -DPREFIX=\"/usr/local\" -DPACKAGE=\"feh\" -DVERSION=\"3.3\"   -c -o imlib.o imlib.c
imlib.c:545:12: warning: ‘curl_quit_function’ defined but not used [-Wunused-function]
 static int curl_quit_function(void *clientp,  curl_off_t dltotal,  curl_off_t dlnow, curl_off_t ultotal, curl_off_t ulnow)
            ^
```
The `curl_quit_function` code was added in response to pull [#435](https://github.com/derf/feh/pull/435)

In issue [#485](https://github.com/derf/feh/issues/485) a fellow CentOS 7 user had an error building feh because CentOS 7 is locked into an old version of libcurl. In the fix, a version guard was wrapped around the `curl_easy_setopt` call, but the rest of the code was unchanged.

Since I don't want to maintain a local build of libcurl, I looked at the curl docs and noticed there is an older callback which serves the same purpose: https://curl.haxx.se/libcurl/c/CURLOPT_PROGRESSFUNCTION.html

The difference between `PROGRESS` and `XFERINFO` is the callback's argument types, with `PROGRESS` using `double` and `XFERINFO` using `curl_off_t`: https://curl.haxx.se/libcurl/c/CURLOPT_XFERINFOFUNCTION.html

The callback's return value logic and use of `CURLOPT_NOPROGRESS` is the same.

For context, the latest libcurl RPM I'm getting from yum updates is `libcurl-7.29.0-54.el7_7.2.x86_64`. The "stable" versions of other distros may encounter similar issues. The CentOS 7 "End of Life" date is 2024-06-30 so you should hear the end of this by then, at least from us pesky CentOS users.

Closes #506

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>