Standardize darwin's lm_info

Standardize darwin's lm_info

Darwin's lm_info structure is used a little bit differently than the
other solib implementations.  The other implementations first allocate
an so_list object, then instanciate their specific lm_info structure,
and assign it to so_list::lm_info.

The Darwin implementation allocates both at the same time
(darwin_so_list).  This patch changes it to be like the others, so that
we'll be able to do some generalizations later.

gdb/ChangeLog:

	* solib-darwin.c (struct darwin_so_list): Remove.
	(darwin_current_sos): Allocate an so_list object instead of a
	darwin_so_list, separately allocate an lm_info object.
	(darwin_free_so): Free lm_info.