loader: Fix startup issues.

* Merge the preload and load stages, use load ordering to try preload's
  first.  This fixes an issue where `preload=res_config_curl` would fail
  unless res_curl and func_curl were also preloaded.  Now it is only
  required that those modules be loaded during startup: autoload or
  regular load is good enough.
* The configuration option `require` and `preload-require` were only
  effective if the modules failed to load.  These options will now abort
  Asterisk startup if required modules fail to reach the 'Running'
  state.
* Missing or invalid 'module.conf' did not prevent startup.  Asterisk
  doesn't do anything without modules so this a fatal error.

Change-Id: Ie4176699133f0e3a823b43f90c3348677e43a5f3