Reimplement unnamed POSIX semaphores using user_mutex

* Fixes sharing semantics, so non-shared semaphores in non-shared
  memory do not become shared after a fork.
* Adds two new system calls: _user_mutex_sem_acquire/release(),
  which reuse the user_mutex address-hashed wait mechanism.
* Named semaphores continue to use traditional sem_id semaphores.

headers/posix/semaphore.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2008-2012 Haiku, Inc.
+ * Copyright 2008-2015 Haiku, Inc.
  * Distributed under the terms of the MIT License.
  */
 #ifndef _SEMAPHORE_H_
@@ -13,8 +13,12 @@
 
 
 typedef struct _sem_t {
-	int32_t	id;
-	int32_t	_padding[3];
+	int32_t type;
+	union {
+		int32_t named_sem_id;
+		int32_t unnamed_sem;
+	} u;
+	int32_t padding[2];
 } sem_t;
 
 #define SEM_FAILED	((sem_t*)(long)-1)

headers/private/kernel/user_mutex.h

@@ -20,6 +20,9 @@ status_t	_user_mutex_lock(int32* mutex, const char* name, uint32 flags,
 status_t	_user_mutex_unlock(int32* mutex, uint32 flags);
 status_t	_user_mutex_switch_lock(int32* fromMutex, int32* toMutex,
 				const char* name, uint32 flags, bigtime_t timeout);
+status_t	_user_mutex_sem_acquire(int32* sem, const char* name, uint32 flags,
+				bigtime_t timeout);
+status_t	_user_mutex_sem_release(int32* sem);
 
 #ifdef __cplusplus
 }

headers/private/system/syscalls.h

@@ -79,6 +79,9 @@ extern status_t		_kern_mutex_lock(int32* mutex, const char* name,
 extern status_t		_kern_mutex_unlock(int32* mutex, uint32 flags);
 extern status_t		_kern_mutex_switch_lock(int32* fromMutex, int32* toMutex,
 						const char* name, uint32 flags, bigtime_t timeout);
+extern status_t		_kern_mutex_sem_acquire(int32* sem, const char* name,
+						uint32 flags, bigtime_t timeout);
+extern status_t		_kern_mutex_sem_release(int32* sem);
 
 /* sem functions */
 extern sem_id		_kern_create_sem(int count, const char *name);