Add linux-compatible memfd_create

memfd_create is effectively a SHM_ANON shm_open(2) mapping with optional
CLOEXEC and file sealing support. This is used by some mesa parts, some
linux libs, and qemu can also take advantage of it and uses the sealing to
prevent resizing the region.

This reimplements shm_open in terms of shm_open2(2) at the same time.

shm_open(2) will be moved to COMPAT12 shortly.

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

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

@@ -182,6 +182,30 @@
  */
 #define	SHM_ALLOW_SEALING		0x00000001
 
+/*
+ * Flags for memfd_create().
+ */
+#define	MFD_ALLOW_SEALING		0x00000001
+#define	MFD_CLOEXEC			0x00000002
+
+/* UNSUPPORTED */
+#define	MFD_HUGETLB			0x00000004
+
+#define	MFD_HUGE_MASK			0xFC000000
+#define	MFD_HUGE_SHIFT			26
+#define	MFD_HUGE_64KB			(16 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_512KB			(19 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_1MB			(20 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_2MB			(21 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_8MB			(23 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_16MB			(24 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_32MB			(25 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_256MB			(28 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_512MB			(29 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_1GB			(30 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_2GB			(31 << MFD_HUGE_SHIFT)
+#define	MFD_HUGE_16GB			(34 << MFD_HUGE_SHIFT)
+
 #endif /* __BSD_VISIBLE */
 
 /*
@@ -242,6 +266,9 @@ int	munlockall(void);
 int	shm_open(const char *, int, mode_t);
 int	shm_unlink(const char *);
 #endif
+#if __BSD_VISIBLE
+int	memfd_create(const char *, unsigned int);
+#endif
 __END_DECLS
 
 #endif /* !_KERNEL */