Remove exec_filename macro

Remove exec_filename macro

This removes the exec_filename macro, replacing it with uses of the
member of current_program_space.  This also renames that member, and
changes it to be a unique pointer.

gdb/ChangeLog
2020-10-29  Tom Tromey  <tom@tromey.com>

	* progspace.h (struct program_space) <exec_filename>: Rename from
	pspace_exec_filename.  Now a unique_xmalloc_ptr.
	* inferior.c (print_selected_inferior): Update.
	(print_inferior): Update.
	* mi/mi-main.c (print_one_inferior): Update.
	* exec.h (exec_filename): Remove macro.
	* corefile.c (get_exec_file): Update.
	* exec.c (exec_close): Update.
	(exec_file_attach): Update.
	* progspace.c (clone_program_space): Update.
	(print_program_space): Update.