Quit curl cleanly even if libcurl is old.

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.

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