base-filesystem: add new helper base_filesystem_create_fd() that operates on an fd, instead of a path

This also changes the open flags from
O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC|O_NOFOLLOW to
O_DIRECTORY|O_CLOEXEC. O_RDONLY is redundant, since O_RDONLY is zero
anyway, and O_DIRECTORY pins the acces mode enough: it doesn't allow
read()/write() anyway when specified. O_NONBLOCK is also pointless given
that O_DIRECTORY is specified, it has no meaning on directories. (It is
useful if we don't know much about the inode we are opening, and could
be a device node or fifo, but the O_DIRECTORY excludes that case.)

O_NOFOLLOW is dropped since there's really no point in blocking out the
initial entrypoint being a symlink. Once we pinned the the root of the
tree it might make sense to restrict symlink use below it, but for the
entrypoint itself it doesn't matter.

src/shared/switch-root.c

@@ -94,7 +94,7 @@ int switch_root(const char *new_root,
         /* Do not fail if base_filesystem_create() fails. Not all switch roots are like base_filesystem_create() wants
          * them to look like. They might even boot, if they are RO and don't have the FS layout. Just ignore the error
          * and switch_root() nevertheless. */
-        (void) base_filesystem_create(new_root, UID_INVALID, GID_INVALID);
+        (void) base_filesystem_create_fd(new_root_fd, new_root, UID_INVALID, GID_INVALID);
 
         if (fchdir(new_root_fd) < 0)
                 return log_error_errno(errno, "Failed to change directory to %s: %m", new_root);