avoid some silly warnings from gcc 4.4 and 4.5.

unlock a dlib::mutex it doesn't own.  This is technically a non-backwards
compatible change to the API but it has always been implemented in a way that
didn't allow this kind of double unlock() on non-Windows platforms.
Additionally, the recent change to the dlib::mutex implementation on windows
also no longer allows this kind of double unlock.

instead of mutex::unlock().

of a "mutex" (i.e. the thing made by CreateMutex()) since a critical section does the
same thing but faster.

function objects.  In general, I noted that in most cases they are safe to use
from multiple threads so long as they are not modified.

violation of the strict aliasing rule.  This fixes a bug which caused
find_max_factor_graph_nmplp() to not work properly on some compilers.