semaphore.h: Add SEM_VALUE_MAX.

It's specified in POSIX, and FreeBSD puts it in this file.

Distinguish it from _POSIX_SEM_VALUE_MAX (which is really a
"minimum value this can have" as specified in POSIX.)

src/system/kernel/posix/realtime_sem.cpp

@@ -679,7 +679,7 @@ _user_realtime_sem_open(const char* userName, int openFlagsOrShared,
 	if (context == NULL)
 		return B_NO_MEMORY;
 
-	if (semCount > MAX_POSIX_SEM_VALUE)
+	if (semCount > SEM_VALUE_MAX)
 		return B_BAD_VALUE;
 
 	// userSem must always be given

src/system/libroot/posix/unistd/conf.cpp

@@ -8,6 +8,7 @@
 
 #include <errno.h>
 #include <limits.h>
+#include <semaphore.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -96,7 +97,7 @@ __sysconf(int name)
 			// unlimited, instead of _POSIX_SEM_NSEMS_MAX
 			return -1;
 		case _SC_SEM_VALUE_MAX:
-			return _POSIX_SEM_VALUE_MAX;
+			return SEM_VALUE_MAX;
 		case _SC_IOV_MAX:
 			return IOV_MAX;
 		case _SC_NPROCESSORS_CONF: