Remove support for LinuxThreads and vendor 2.4 kernels w/ backported NPTL

Remove support for LinuxThreads and vendor 2.4 kernels w/ backported NPTL

Since we now rely on PTRACE_EVENT_CLONE being available (added in
Linux 2.5.46), we're relying on NPTL.

This commit removes the support for older LinuxThreads, as well as the
workarounds for vendor 2.4 kernels with NPTL backported.

 - Rely on tkill being available.

 - Assume gdb doesn't get cancel signals.

 - Remove code that checks the LinuxThreads restart and cancel signals
   in the inferior.

 - Assume that __WALL is available.

 - Assume that non-leader threads report WIFEXITED.

 - Thus, no longer need to send signal 0 to check whether threads are
   still alive.

 - Update comments throughout.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/ChangeLog:

	* configure.ac: Remove tkill checks.
	* configure, config.in: Regenerate.
	* linux-nat.c: Remove HAVE_TKILL_SYSCALL check.  Update top level
	comments.
	(linux_nat_post_attach_wait): Remove 'cloned' parameter.  Use
	__WALL.
	(attach_proc_task_lwp_callback): Don't set the cloned flag.
	(linux_nat_attach): Adjust.
	(kill_lwp): Remove HAVE_TKILL_SYSCALL check.  No longer fall back
	to 'kill'.
	(linux_handle_extended_wait): Use __WALL.  Don't set the cloned
	flag.
	(wait_lwp): Use __WALL.  Update comments.
	(running_callback, stop_and_resume_callback): Delete.
	(linux_nat_filter_event): Don't stop and resume all lwps. Don't
	check if the event LWP has previously exited.
	(check_zombie_leaders): Update comments.
	(linux_nat_wait_1): Use __WALL.
	(kill_wait_callback): Don't handle clone processes separately.
	Use __WALL instead.
	(linux_thread_alive): Delete.
	(linux_nat_thread_alive): Return true as long as the LWP is in the
	LWP list.
	(linux_nat_update_thread_list): Assume the kernel supports
	PTRACE_EVENT_CLONE.
	(get_signo): Delete.
	(lin_thread_get_thread_signals): Remove LinuxThreads references.
	No longer check __pthread_sig_restart / __pthread_sig_cancel in
	the inferior.
	* linux-nat.h (struct lwp_info) <cloned>: Delete field.
	* linux-thread-db.c: Update comments.
	(_initialize_thread_db): Remove LinuxThreads references.
	* nat/linux-waitpid.c (my_waitpid): No longer emulate __WALL.
	Pass down flags unmodified.
	* linux-waitpid.h (my_waitpid): Update documentation.

gdb/gdbserver/ChangeLog:

	* linux-low.c (linux_kill_one_lwp): Remove references to
	LinuxThreads.
	(kill_lwp): Remove HAVE_TKILL_SYSCALL check.  No longer fall back
	to 'kill'.
	(linux_init_signals): Delete.
	(initialize_low): Adjust.
	* thread-db.c (thread_db_init): Remove LinuxThreads reference.