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/mountfsd/mountwork.c

@@ -1,9 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <sys/mount.h>
-#if WANT_LINUX_FS_H
-#include <linux/fs.h>
-#endif
 #include <linux/loop.h>
 
 #include "sd-daemon.h"
@@ -20,7 +17,6 @@
 #include "io-util.h"
 #include "json-util.h"
 #include "main-func.h"
-#include "missing_mount.h"
 #include "missing_syscall.h"
 #include "namespace-util.h"
 #include "nsresource.h"