* linux-fork.c (forks_exist_p): Drop unnecessary extern.

	* linux-fork.c (forks_exist_p): Drop unnecessary extern.
	(add_fork, linux_fork_killall, linux_fork_mourn_inferior)
	(linux_fork_detach): Ditto.
	(delete_fork_command): Rename to ...
	(delete_checkpoint_command): ... this.
	(detach_fork_command): Rename to ...
	(detach_checkpoint_command): ... this.
	(info_forks_command): Rename to ...
	(info_checkpoints_command): ... this.
	(_initialize_linux_fork): Remove stale comments.  Adjust.