Add objfile-progspace to Guile interface

Add objfile-progspace to Guile interface

This commit adds an objfile-progspace accessor to the (gdb) Guile
module.

gdb/testsuite/ChangeLog:

	* gdb.guile/scm-objfile.exp: Add objfile-progspace test.

gdb/doc/ChangeLog:

	* guile.texi (Objfiles In Guile): Document objfile-progspace.

gdb/ChangeLog:

	* guile/scm-objfile.c (gdbscm_objfile_progspace): New function.
	(objfile_functions): Bind gdbscm_objfile_progspace to
	objfile-progspace.
	* guile/lib/gdb.scm: Add objfile-progspace to exports.