Closes: #124

Partial fix of: #310

Fixes: #415

Trying to handle stdin when select() in fact returned an error due to being
interrupted by a signal is a rather bad idea.

See issue #405

See issue #405

See issue #405

Closes #400

Closes #279

Addresses part of issue #75

This restores pre-2.21 ~/.fehbg behaviour. Replicating the entire commandline
in ~/.fehbg turned out to do more harm than good.

Closes #398

--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.

Fixes non-random behaviour when randomizing file lists several times per
second.

Closes #349