core: add new init_script_path option:

`init_script_path` option can be used in `[DEFAULTS]` to run a script at the end of
backend initialisation. A new `init_pre_script` method is used to wait for backend to
reach this stage (designed to be used mostly by CLI frontend), then the usual `ready_get`
method is finished once the script is finished.

libervia/backend/tools/common/async_process.py

@@ -133,7 +133,9 @@ class CommandProtocol(protocol.ProcessProtocol):
         cmd_args = [command] + args
         if "env" not in kwargs:
             # we pass parent environment by default
-            kwargs["env"] = None
+            # FIXME: `None` doesn't seem to work, despite what documentation says, to be
+            #    checked and reported upstream if confirmed.
+            kwargs["env"] = os.environ
         reactor.spawnProcess(prot,
                              command,
                              cmd_args,