Change exec_close to be a method on program_space

Change exec_close to be a method on program_space

exec_close uses the current program space, so it seemed cleaner to
change it to be a method on program_space.  This patch makes this
change.

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

	* progspace.c (program_space::exec_close): New method, from
	exec_close in exec.c.
	* exec.c (exec_close): Move to progspace.c.
	(exec_target::close, exec_file_attach): Update.
	* progspace.h (struct program_space) <exec_close>: Declare
	method.