core: move several source files to src/shared

As, the files are used by many executables, fstab-generator, remount-fs,
machine-id-setup, and etc.
With this change, the total size of the relevant executables and
libshared slightly decreases.

Before:
```
$ ll systemd-fstab-generator systemd-remount-fs systemd-machine-id-setup systemd-shutdown systemd-nspawn systemd src/shared/libsystemd-shared-247.so
-rwxrwxr-x 1 watanabe watanabe 7577800 Jan  5 13:35 src/shared/libsystemd-shared-247.so
-rwxrwxr-x 1 watanabe watanabe 4608360 Jan  5 13:35 systemd
-rwxrwxr-x 1 watanabe watanabe  117240 Jan  5 13:35 systemd-fstab-generator
-rwxrwxr-x 1 watanabe watanabe   61576 Jan  5 13:35 systemd-machine-id-setup
-rwxrwxr-x 1 watanabe watanabe  853080 Jan  5 13:35 systemd-nspawn
-rwxrwxr-x 1 watanabe watanabe   70600 Jan  5 13:35 systemd-remount-fs
-rwxrwxr-x 1 watanabe watanabe  172624 Jan  5 13:35 systemd-shutdown
```
Total: 13461280

After:
```
$ ll systemd-fstab-generator systemd-remount-fs systemd-machine-id-setup systemd-shutdown systemd-nspawn systemd src/shared/libsystemd-shared-247.so
-rwxrwxr-x 1 watanabe watanabe 7658336 Jan  5 13:32 src/shared/libsystemd-shared-247.so
-rwxrwxr-x 1 watanabe watanabe 4523560 Jan  5 13:32 systemd
-rwxrwxr-x 1 watanabe watanabe   78288 Jan  5 13:32 systemd-fstab-generator
-rwxrwxr-x 1 watanabe watanabe   30984 Jan  5 13:32 systemd-machine-id-setup
-rwxrwxr-x 1 watanabe watanabe  840384 Jan  5 13:32 systemd-nspawn
-rwxrwxr-x 1 watanabe watanabe   39104 Jan  5 13:32 systemd-remount-fs
-rwxrwxr-x 1 watanabe watanabe  117160 Jan  5 13:32 systemd-shutdown
```
Total: 13287816

1 files changed, 12 insertions, 0 deletions

diff --git a/src/shared/mount-setup.h b/src/shared/mount-setup.h
new file mode 100644
index 0000000000..29bd62f6bf
--- /dev/null
+++ b/src/shared/mount-setup.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stdbool.h>
+
+int mount_setup_early(void);
+int mount_setup(bool loaded_policy, bool leave_propagation);
+
+int mount_cgroup_controllers(void);
+
+bool mount_point_is_api(const char *path);
+bool mount_point_ignore(const char *path);