include/ipc: Add functions to ipc interface so it can be extended

include/ipc: Add functions to ipc interface so it can be extended

The ipc module provides an interface, but it cannot meaningfully
be extended because the interface is missing functions that need to
be overridden by an extending module.

This commit makes several message-related functions public and adds
them to the interface that is provided by the ipc module. This
further adds the `ipc_get_root()' function, which replaces the
`__ipc_root' variable, allowing modules that extend the interface to
use a different root directory.