asterisk-16.x: update init and configuration

asterisk-16.x: update init and configuration

This commit changes handling of configuration and init a bit:

1. Asterisk configuration files are installed with $(INSTALL_DATA). This
   way there is no need to chown /etc/asterisk to user asterisk. The
   package provides the standard asterisk configuration (no secrets) so
   it's OK to install it readable for all. If users put something
   sensitive in there they can update permissions how they like.
2. The init script no longer creates directories that the users can
   configure in /etc/config/asterisk, which could be all kinds of
   strange directories. Instead only the default directories are
   created, and only the ones that reside on volatile memory. The other
   directories are now created by the package itself.
3. This commit removes the ability to choose a user/group via
   /etc/config/asterisk. This makes the init script simpler. If anybody
   wants to play around with the user/group, then it's up to them to fix
   the permissions.
4. "local" is removed because it's not POSIX.
5. From issue #520 we know that adding the user to the dialout group
   does not help with the device permissions for chan_lantiq. This
   removes it again.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>