Make "checkpoint" not rely on inferior_ptid

Make "checkpoint" not rely on inferior_ptid

Don't rely on "inferior_ptid" deep within add_fork.  In the
multi-target branch, I'm forcing inferior_ptid to null_ptid early in
infrun event handling to make sure we don't inadvertently rely on the
current thread/target when we shouldn't, and that caught some bad or
unnecessary assumptions throughout.

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

	* linux-fork.c (new_fork): New, split out of ...
	(add_fork): ... this.  Return void.  Move "first fork" special
	case from here, to ...
	(checkpoint_command): ... here.
	* linux-linux.h (add_fork): Return void.