basic: introduce our own sys/mount.h implementation

To resolve conflict with sys/mount.h and linux/mount.h or linux/fs.h.

The conflict between sys/mount.h and linux/mount.h is resolved in
glibc-2.37 (774058d72942249f71d74e7f2b639f77184160a6), but our baseline
is still glibc-2.31. Also, even with the version or newer, still
sys/mount.h conflicts with linux/fs.h, which is included by
linux/btrfs.h.

This introduces our own implementation of sys/mount.h, that can be
simultaneously included with linux/mount.h and linux/fs.h. This also
imports linux/fs.h, linux/mount.h, and several other dependent headers.
The introduced sys/mount.h header itself may not be enough simple, but
by using the header, we can drop most of workarounds in other source files.

src/tmpfiles/tmpfiles.c

@@ -5,7 +5,6 @@
 #include <fnmatch.h>
 #include <getopt.h>
 #include <limits.h>
-#include <linux/fs.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdlib.h>
@@ -45,6 +44,7 @@
 #include "log.h"
 #include "macro.h"
 #include "main-func.h"
+#include "missing_fs.h"
 #include "missing_syscall.h"
 #include "mkdir-label.h"
 #include "mount-util.h"