follow-fork: don't lose the ptids as set by the target

This Linuxism has made its way into infrun.c, in the follow-fork code:

       inferior_ptid = ptid_build (child_pid, child_pid, 0);

The OS-specific code should fill in the LWPID, TID parts with the
appropriate values, if any, and the core code should not be peeking at
the components of the ptids.

gdb/
2015-03-04  Pedro Alves  <palves@redhat.com>

	* infrun.c (follow_fork_inferior): Use the whole of the
	inferior_ptid and pending_follow.related_pid ptids instead of
	building ptids from the process components.  Adjust verbose output
	to use target_pid_to_str.
	* linux-nat.c (linux_child_follow_fork): Use the whole of the
	inferior_ptid and pending_follow.related_pid ptids instead of
	building ptids from the process components.