Partial fix of: #310

Addresses part of issue #75

--keep-zoom-vp will no longer block the dynamic window resizing mechanism.

Passing an empty string to the --geometry option will enable fixed geometry mode without having to specify anything else

This simplifies the logic behind the automatic zoom ratio calculation, which is used by both --auto-zoom and --scale-down.

See #278

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.

This will prevent unnecessary calls to 'feh_calc_needed_zoom'

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/

Closes #351

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 fixes a segfault when combining --scale-down and --thumbnails on a
sufficiently large directory

this actually uses the old "force geometry" workaround to get the
scale down behaviour (which makes the code a lot cleaner) by
first setting the geometry after the window is created (to avoid
creating a 0x0 sized window, which X does not like).