Closes #218

Closes: #382, #384

This introduces a new feh_should_ignore_image function which is called
at appropriate places in those modes to skip images which are loadable but
undesired.

Closes #381

* Prevents nasty loading loops

* Prevents zombie subprocesses

* Fixes the conversion timeout detection routine

Halves the start-up time of the slideshow if the title contains data from 'file->info'

This will (by default) check the image dimension on-the-fly, but you can still do it beforehand by specifying the preload flag.

On glibc, if output is redirected to a file, output will look like this:

touch test/ok/gif
touch test/ok/jpg
touch test/ok/png
touch test/ok/pnm
test/ok/gif
test/ok/jpg
test/ok/png
test/ok/pnm

On musl, if stdout is redirected to a file, output looks like this:

test/ok/gif
touch test/ok/gif
touch test/ok/jpg
touch test/ok/png
touch test/ok/pnm
test/ok/jpg
test/ok/png
test/ok/pnm

On glibc and musl, if stdout is interactive, it looks like this:

test/ok/gif
touch test/ok/gif
test/ok/jpg
touch test/ok/jpg
test/ok/png
touch test/ok/png
test/ok/pnm
touch test/ok/pnm

Adding two fflush calls makes all behavior look like the last example.
Test cases have been updated accordingly.

Fixes issue 325.

This will prevent unnecessary calls to 'feh_calc_needed_zoom'

This option allows you to change the default imlib2 image cache size of 4 MiB.

This prevents removing the image data from the cache, when moving back and forth between images. As suggested by the Imlib documentation: http://alien.cern.ch/cache/imlib2-1.0.6/doc/

might resolve #333 and #348

Closes #351

Similar to the `curl` command-line tool.

This fixes --force-aliasing (and possibly other options) missing from the file.

Happens e.g. when using -O without enabling thumbnail/index mode

Closes #335

The check if buffer == NULL is always false, since buffer is an
autoamtic variable allocated when entering the function.  What we
instead want to do is to check if the string is empty after the call to
exif_get_info(), since that means we could not read any exif
information.
When the code once more is enabled, I discovered that we need to copy
the information string into info_buf as well as into  buffer, since it
is the former that is used to print the exif information on top of the
picture.  Without this change, imlib warns about trying to write NULL
strings.

Closes #326

When feh loses its controlling terminal at runtime, e.g. due to backgrounding /
disowning, it will no longer issue a warning on each terminal keystroke.