capture.cpp: allsky_common
capture.cpp: allsky_common

* Remove #defines and functions that were moved to allsky_common.cpp and allsky_common.h
* Add debugging code to check if actual time to take exposure is close to requested exposure time.
* Call doOverlay() to add text to the image overlay; a future commit will replace the old code that did this.
* Centralize calling check_max_errors() so it's only in one place.
* Rename showHistogram to showMean to match what's in capture_RPiHQ.cpp
thomasjacquin/allsky,thomasjacquin/allsky,thomasjacquin/allsky