Implement read(2)/write(2) and neccessary lseek(2)

for posix shmfd. Add MAC framework entries for posix shm read and write.

Do not allow implicit extension of the underlying memory segment past
the limit set by ftruncate(2) by either of the syscalls.  Read and
write returns short i/o, lseek(2) fails with EINVAL when resulting
offset does not fit into the limit.

Discussed with:	alc
Tested by:	pho
Sponsored by:	The FreeBSD Foundation

newlib/libc/sys/rtems/include/sys/mman.h

@@ -190,6 +190,10 @@ typedef	__size_t	size_t;
 #endif
 
 #if defined(_KERNEL) || defined(_WANT_FILE)
+#include <sys/lock.h>
+#include <sys/mutex.h>
+#include <sys/queue.h>
+#include <sys/rangelock.h>
 #include <vm/vm.h>
 
 struct file;
@@ -215,6 +219,9 @@ struct shmfd {
 
 	struct label	*shm_label;		/* MAC label */
 	const char	*shm_path;
+
+	struct rangelock shm_rl;
+	struct mtx	shm_mtx;
 };
 #endif