logging: Add lxc_log_options_no_override function

In current LXC, loglevel and logfile are write-once functions.
That behaviour was appropriate when those two were first introduced
(pre-API) but with current API, one would expect to be able to
set_config_item those multiple times.

So instead, introduce lxc_log_options_no_override which when called
turns those two config keys read-only and have all existing binaries
which use log_init call that function once they're done setting the
value requested by the user.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>