Add a shm_open2 syscall to support upcoming memfd_create

shm_open2 allows a little more flexibility than the original shm_open.
shm_open2 doesn't enforce CLOEXEC on its callers, and it has a separate
shmflag argument that can be expanded later. Currently the only shmflag is
to allow file sealing on the returned fd.

shm_open and memfd_create will both be implemented in libc to use this new
syscall.

__FreeBSD_version is bumped to indicate the presence.

Reviewed by:	kib, markj
Differential Revision:	https://reviews.freebsd.org/D21393

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

@@ -176,6 +176,12 @@
  * Anonymous object constant for shm_open().
  */
 #define	SHM_ANON		((char *)1)
+
+/*
+ * shmflags for shm_open2()
+ */
+#define	SHM_ALLOW_SEALING		0x00000001
+
 #endif /* __BSD_VISIBLE */
 
 /*