This patch implements two new MI notifications,
=library-loaded and =library-unloaded. There's just one point of note --
identifications of shared libraries. In order for frontend to maintain
accurate list of shared libraries using notifications, there should be
a reliable way of matching unloaded library, as reported by =library-unloaded,
with a previously loaded library. Currently, GDB keeps two names associated
with a library, so_original_name which is reported by the target and so_name,
which is basically the name of the file where symbols are read from.

All internal comparisons of libraries for equality uses so_original_name, so
it makes sense that frontend use it too. However, it is technically possible
to load the same shared library twice, so ideally we use something more unique --
but this is extra work. So, MI notifications output three fields

	- id (documented as opaque)
	- target name
	- host name

presently, so_original_name is used for the first two, but in future, id might
become numeric, or it might look like libfoo.so@2. 

Comments? Is solib.c change OK? Are doc changes OK?

- Volodya