Refactor dbprint to enable selective debug output

Trying to debug LibVMI with the debug output currently is quite a challenge considering the verbosity of the debug output. While generally it is OK to just run the app with all debug output piped into a file and grepping later for the type of debug output that one is interested in, with events that debugging flow is quite inconvenient. 

With this PR dbprint is refactored as to take a debug category as the first input so that one can select which output to get printed. This is only a first pass, haven't categorized everything appropriately so far (there is a bunch that are still under VMI_DEBUG_MISC).

RFC