- 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>

include/sys/ucontext.h

@@ -0,0 +1,22 @@
+#ifndef _SYS_UCONTEXT_H
+#define _SYS_UCONTEXT_H 1
+
+#include <signal.h>
+#include <machine/mcontext.h>
+
+#define NCARGS 6
+
+#define UCF_SWAPPED	001 /* Context has been swapped in by swapcontext(3) */
+#define UCF_IGNFPU	002 /* Ignore FPU context by get or setcontext(3) */
+#define UCF_IGNSIGM	004 /* Ignore signal mask by get or setcontext(3) */
+typedef struct __ucontext ucontext_t;
+struct __ucontext {
+  unsigned int uc_flags;  /* Properties of ucontext */
+  ucontext_t *uc_link;    /* Next context to resume when current is finished */
+  mcontext_t uc_mcontext; /* Machine state */
+  sigset_t uc_sigmask;    /* Signals blocked in this context */
+  stack_t uc_stack;       /* The stack used by this context */
+};
+
+#endif /* _SYS_UCONTEXT_H */
+