repart: when copying files into vfat or similar, do not set ownership

$ mkdir /var/tmp/files
$ touch /var/tmp/files/a
$ mkdir /var/tmp/conf
$ cat >>/var/tmp/conf/esp.conf
[Partition]
Type=esp
Format=vfat
CopyFiles=/var/tmp/files:/
$ truncate /var/tmp/disk -s 300M
$ sudo systemd-repart --dry-run=no --empty=require --definitions=/var/tmp/conf /var/tmp/disk
...
Populating vfat filesystem.
Failed to copy '...' to '/run/systemd/mount-root/': Operation not permitted
(sd-copy) failed with exit status 1.

The issue is that if there's a file owned by non-root and we try to copy
it into a newly-created DOS partition, fchown fails:
  fchown(11</run/systemd/mount-root/...>, 1000, 1000) = -1 EPERM (Operation not permitted)
We want to ignore file ownership in such cases, so pass our own UID/GID
to copy_tree_at(), which turns the fchown into a noop and let's the
operation pass through.

Fixes #38863.

src/basic/mountpoint-util.c

@@ -485,6 +485,22 @@ bool fstype_can_fmask_dmask(const char *fstype) {
         return streq(fstype, "vfat") || (mount_option_supported(fstype, "fmask", "0177") > 0 && mount_option_supported(fstype, "dmask", "0077") > 0);
 }
 
+bool fstype_can_ownership(const char *fstype) {
+        /* File systems which are not known to not support uid/gid ownership.
+         * For some types, this can be a bit murky. So just exclude the ones that for sure
+         * don't support with the current implementations in Linux. */
+
+        return !STR_IN_SET(ASSERT_PTR(fstype),
+                           "adfs",
+                           "exfat",
+                           "fat",
+                           "hfs",
+                           "hpfs",
+                           "msdos",
+                           "ntfs",
+                           "vfat");
+}
+
 bool fstype_can_uid_gid(const char *fstype) {
         /* All file systems that have a uid=/gid= mount option that fixates the owners of all files and
          * directories, current and future. Note that this does *not* ask the kernel via

src/basic/mountpoint-util.h

@@ -66,6 +66,7 @@ bool fstype_is_api_vfs(const char *fstype);
 bool fstype_is_blockdev_backed(const char *fstype);
 bool fstype_is_ro(const char *fsype);
 bool fstype_can_discard(const char *fstype);
+bool fstype_can_ownership(const char *fstype);
 bool fstype_can_uid_gid(const char *fstype);
 bool fstype_can_fmask_dmask(const char *fstype);