Throughout: mop up all the Doxygen warnings and undocumented
	stuff.

	* dbus/dbus-sysdeps.c (do_exec): do not use execvp, we don't want
	to search any paths.

	* dbus/dbus-threads.c: move global mutex initializers to
	dbus-internals.h, multiple prototypes was confusing doxygen
	besides being kind of ugly

	* Doxyfile (PREDEFINED): have Doxygen define
	DOXYGEN_SHOULD_SKIP_THIS so we can exclude things from
	docs with #ifndef DOXYGEN_SHOULD_SKIP_THIS
	(do not abuse the feature! it's for stuff like the autogenerated
	macros in dbus-md5.c, not just for things you don't feel like
	documenting...)