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/core/execute.c

@@ -14,8 +14,6 @@
 #include <unistd.h>
 #include <utmpx.h>
 
-#include <linux/fs.h> /* Must be included after <sys/mount.h> */
-
 #include "sd-messages.h"
 
 #include "af-list.h"

src/core/load-fragment.c

@@ -5,7 +5,6 @@
 
 #include <errno.h>
 #include <fcntl.h>
-#include <linux/fs.h>
 #include <linux/oom.h>
 #include <sched.h>
 #include <sys/resource.h>
@@ -48,6 +47,7 @@
 #include "limits-util.h"
 #include "load-fragment.h"
 #include "log.h"
+#include "missing_fs.h"
 #include "mountpoint-util.h"
 #include "nulstr-util.h"
 #include "open-file.h"

src/core/namespace.c

@@ -7,9 +7,6 @@
 #include <sys/file.h>
 #include <sys/mount.h>
 #include <unistd.h>
-#if WANT_LINUX_FS_H
-#include <linux/fs.h>
-#endif
 
 #include "alloc-util.h"
 #include "base-filesystem.h"