- Add support for the ucontext system calls (getcontext, setcontext,

swapcontext, and makecontext).
- Fix VM to not erroneously think the stack segment and data segment have
  collided when a user-space thread invokes brk().
- Add test51 to test ucontext functionality.
- Add man pages for ucontext system calls.

include/sys/Makefile

@@ -8,7 +8,7 @@ INCS=	asynchio.h dir.h file.h ioc_cmos.h ioc_disk.h \
 	mount.h mtio.h param.h ptrace.h queue.h resource.h \
 	select.h sem.h shm.h sigcontext.h signal.h socket.h \
 	soundcard.h statfs.h stat.h svrctl.h timeb.h \
-	time.h times.h types.h uio.h un.h utsname.h video.h vm.h \
+	time.h times.h types.h ucontext.h uio.h un.h utsname.h video.h vm.h \
 	wait.h
 
 .include <minix.kinc.mk>