python: Add Inferior.progspace property

python: Add Inferior.progspace property

This patch adds a progspace property to the gdb.Inferior type, which
allows getting the gdb.Progspace object associated to that inferior.
In conjunction with the following patch, this will allow scripts iterate
on objfiles associated with a particular inferior.

gdb/ChangeLog:

	* python/py-inferior.c (infpy_get_progspace): New function.
	(inferior_object_getset): Add progspace property.
	* NEWS: Mention the new property.

gdb/doc/ChangeLog:

	* python.texi (Inferiors In Python): Document
	Inferior.progspace.
	(Program Spaces In Python): Document that
	gdb.current_progspace() is the same as
	gdb.selected_inferior().progspace.

gdb/testsuite/ChangeLog:

	* gdb.python/py-inferior.exp: Add tests for Inferior.progspace
	and a few other Inferior properties when the Inferior is no
	longer valid.