Add rmdirat and renameat to run-as commands

Add support for the rmdirat and renameat commands to the run-as
infrastructure. These commands use the directory_handle compatibility
layer to provide rename and directory deletion relative to a
(or multiple) directory file descriptors.

The rmdirat name is used even though there are no rmdirat syscall
(at least on Linux). The functionality that would be provided by
rmdirat() (vs rmdir) is provided through unlinkat(..., AT_REMOVEDIR).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>