build-sys: use linux/memfd.h if available

linux/memfd.h was added linux 3.17, so it might not be widely
available for a while.

Also, check if memfd_create is defined, for the HAVE_LINUX_MEMFD_H
check to have a chance of succeeding.

Also, collapse all ifdefs for memfd-related stuff, because they
were all added together so there's no need to check separately.

src/shared/memfd.c

@@ -25,11 +25,15 @@
 #include <sys/mman.h>
 #include <sys/prctl.h>
 
+#ifdef HAVE_LINUX_MEMFD_H
+#  include <linux/memfd.h>
+#endif
+
 #include "util.h"
 #include "bus-label.h"
-#include "missing.h"
 #include "memfd.h"
 #include "utf8.h"
+#include "missing.h"
 
 int memfd_new(const char *name) {
         _cleanup_free_ char *g = NULL;

src/shared/missing.h

@@ -65,25 +65,11 @@
 
 #ifndef F_ADD_SEALS
 #define F_ADD_SEALS (F_LINUX_SPECIFIC_BASE + 9)
-#endif
-
-#ifndef F_GET_SEALS
 #define F_GET_SEALS (F_LINUX_SPECIFIC_BASE + 10)
-#endif
 
-#ifndef F_SEAL_SEAL
 #define F_SEAL_SEAL     0x0001  /* prevent further seals from being set */
-#endif
-
-#ifndef F_SEAL_SHRINK
 #define F_SEAL_SHRINK   0x0002  /* prevent file from shrinking */
-#endif
-
-#ifndef F_SEAL_GROW
 #define F_SEAL_GROW     0x0004  /* prevent file from growing */
-#endif
-
-#ifndef F_SEAL_WRITE
 #define F_SEAL_WRITE    0x0008  /* prevent writes */
 #endif