C++ify program_space

This patch makes program_space a C++ object by adding a
constructor/destructor, giving default values to fields, and using
new/delete.

gdb/ChangeLog:

	* progspace.h (struct program_space): Add constructor and
	destructor, initialize fields.
	(add_program_space): Remove.
	* progspace.c (add_program_space): Rename to...
	(program_space::program_space): ... this.
	(release_program_space): Rename to...
	(program_space::~program_space): ... this.
	(delete_program_space): Use delete to delete program_space.
	(initialize_progspace): Use new to allocate program_space.
	* inferior.c (add_inferior_with_spaces): Likewise.
	(clone_inferior_command): Likewise.
	* infrun.c (follow_fork_inferior): Likewise.
	(handle_vfork_child_exec_or_exit): Likewise.