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-serialize.c

@@ -1177,6 +1177,10 @@ static int exec_parameters_serialize(const ExecParameters *p, const ExecContext
         if (r < 0)
                 return r;
 
+        r = serialize_fd(f, fds, "exec-parameters-root-directory-fd", p->root_directory_fd);
+        if (r < 0)
+                return r;
+
         r = serialize_fd(f, fds, "exec-parameters-exec-fd", p->exec_fd);
         if (r < 0)
                 return r;
@@ -1422,6 +1426,16 @@ static int exec_parameters_deserialize(ExecParameters *p, FILE *f, FDSet *fds) {
                                 continue;
 
                         close_and_replace(p->stderr_fd, fd);
+
+                } else if ((val = startswith(l, "exec-parameters-root-directory-fd="))) {
+                        int fd;
+
+                        fd = deserialize_fd(fds, val);
+                        if (fd < 0)
+                                continue;
+
+                        close_and_replace(p->root_directory_fd, fd);
+
                 } else if ((val = startswith(l, "exec-parameters-exec-fd="))) {
                         int fd;
 
@@ -1994,6 +2008,10 @@ static int exec_context_serialize(const ExecContext *c, FILE *f) {
         if (r < 0)
                 return r;
 
+        r = serialize_bool_elide(f, "exec-context-root-directory-as-fd", c->root_directory_as_fd);
+        if (r < 0)
+                return r;
+
         switch (c->std_input) {
         case EXEC_INPUT_NAMED_FD:
                 r = serialize_item(f, "exec-context-std-input-fd-name", c->stdio_fdname[STDIN_FILENO]);
@@ -3000,6 +3018,11 @@ static int exec_context_deserialize(ExecContext *c, FILE *f) {
                         if (r < 0)
                                 return r;
                         c->stdio_as_fds = r;
+                } else if ((val = startswith(l, "exec-context-root-directory-as-fd="))) {
+                        r = parse_boolean(val);
+                        if (r < 0)
+                                return r;
+                        c->root_directory_as_fd = r;
                 } else if ((val = startswith(l, "exec-context-std-input-fd-name="))) {
                         r = free_and_strdup(&c->stdio_fdname[STDIN_FILENO], val);
                         if (r < 0)