Replace SG_GENERAL by more specific log classes in many places.
Allow "," to separate logging classes (using "|" is odd on the
command-line).
Also add new option to make logging more useful for developers.
You can use:
--log-level=debug --log-class=environment
to only get environment debug messages, or
--log-level=debug --log-class=sound,ai
to only get debug messages related to the sound or AI subsystem.
SimGear change. It changes all the SG_xxxx to be the 'real' includes, and gets
rid of many #ifdef SG_HAVE_STD_INCLUDES. As an added bonus, rather than
replacing 'SG_USING_NAMESPACE(std)' with 'using namespace std', I just fixed
the small number of places to use std:: explicitly. So we're no longer polluting
the global namespace with the entire contents of std, in many cases.
There is one more 'mechanical' change to come - getting rid of SG_USING_STD(X),
but I want to keep that separate from everything else. (There's another
mechnical change, replacing <math.h> with <cmath> and so on *everywhere*, but
one step at a time)
