Allow rtprio_thread to operate on threads of any process

This in particular unbreaks rtkit.

The limitation was a leftover of previous state, to quote a
comment:

/*
 * Though lwpid is unique, only current process is supported
 * since there is no efficient way to look up a LWP yet.
 */

Long since then a global tid hash was introduced to remedy
the problem.

Permission checks still apply.

Submitted by:	greg_unrelenting.technology (Greg V)
Differential Revision:	https://reviews.freebsd.org/D27158

sys/kern/kern_resource.c

@@ -315,8 +315,7 @@ sys_rtprio_thread(struct thread *td, struct rtprio_thread_args *uap)
 		td1 = td;
 		PROC_LOCK(p);
 	} else {
-		/* Only look up thread in current process */
-		td1 = tdfind(uap->lwpid, curproc->p_pid);
+		td1 = tdfind(uap->lwpid, -1);
 		if (td1 == NULL)
 			return (ESRCH);
 		p = td1->td_proc;