C++ify fork_info, use std::list

C++ify fork_info, use std::list

- Convert new_fork and free_fork to fork_info ctor/dtor.
- Use std::list.

gdb/ChangeLog:
2019-03-06  Pedro Alves  <palves@redhat.com>

	* linux-fork.c: Include <list>.
	(fork_list): Now a std::list instance.
	(fork_info): Add ctor, dtor, and in-class initialize all fields.
	(forks_exist_p, find_last_fork): Adjust.
	(new_fork): Delete.
	(one_fork_p): New.
	(add_fork): Adjust.
	(free_fork): Delete, folded into fork_info::~fork_info().
	(delete_fork, find_fork_ptid, find_fork_id, find_fork_pid):
	Adjust.
	(init_fork_list): Delete.
	(linux_fork_killall, linux_fork_mourn_inferior)
	(linux_fork_detach, info_checkpoints_command): Adjust.
	(_initialize_linux_fork): No longer call init_fork_list.