core: Add RootDirectoryFileDescriptor=

RootDirectory= but via a open_tree() file descriptor. This allows
setting up the execution environment for a service by the client in
a mount namespace and then starting a transient unit in that execution
environment using the new property.

We also add --root-directory= and --same-root-dir= to systemd-run to
have it run services within the given root directory. As systemd-run
might be invoked from a different mount namespace than what systemd is
running in, systemd-run opens the given path with open_tree() and then
sends it to systemd using the new RootDirectoryFileDescriptor= property.

src/core/execute.h

@@ -218,6 +218,7 @@ typedef struct ExecContext {
         /* At least one of stdin/stdout/stderr was initialized from an fd passed in. This boolean survives
          * the fds being closed. This only makes sense for transient units. */
         bool stdio_as_fds;
+        bool root_directory_as_fd;
 
         char *stdio_fdname[3];
         char *stdio_file[3];
@@ -418,6 +419,7 @@ typedef struct ExecParameters {
         int stdin_fd;
         int stdout_fd;
         int stderr_fd;
+        int root_directory_fd;
 
         /* An fd that is closed by the execve(), and thus will result in EOF when the execve() is done. */
         int exec_fd;
@@ -449,6 +451,7 @@ typedef struct ExecParameters {
                 .stdin_fd               = -EBADF, \
                 .stdout_fd              = -EBADF, \
                 .stderr_fd              = -EBADF, \
+                .root_directory_fd      = -EBADF, \
                 .exec_fd                = -EBADF, \
                 .bpf_restrict_fs_map_fd = -EBADF, \
                 .user_lookup_fd         = -EBADF, \