add /sim/time/real/ (local machine time) analog to /sim/time/utc/

add /sim/time/real/ (local machine time) analog to /sim/time/utc/

localtime() is once commented out in simgear/io/sg_binobj.cxx and
a warning says the it can screw up sim time. This is only because
localtime() isn't thread-safe (and its thread-safe cousin localtime_r()
isn't portable). As long as all timing is done in the main thread
(which is the case) there should be no problem. Otherwise we'd have
to implement a simple mutex locked wrapper.