gdb/guile: Have gdbscm_safe_source_script return a unique_ptr

Change gdbscm_safe_source_script to return a
gdb::unique_xmalloc_ptr<char> instead of a raw char*.  Update the
users of this function.

There should be no user visible change after this commit.

gdb/ChangeLog:

	* guile/guile-internal.h (gdbscm_safe_source_script): Change
	function return type.
	* guile/guile.c (gdbscm_source_script): Update to handle change in
	gdbscm_safe_source_script.
	* guile/scm-objfile.c (gdbscm_source_objfile_script): Likewise.
	* guile/scm-safe-call.c (gdbscm_safe_source_script): Change return
	type.