usernsexec: init log fd

usernsexec: init log fd

lxc-usernsexec uses some functions (e.g. lxc_map_ids()), which are part of
the lxc library and thus use the WARN etc. macros to emit log messages.
However, it doesn't initialize the log in any way, so these messages go
into the ether.

lxc-usernsexec currently has no log parameters, so let's just log these to
stderr. Someone can do something fancier later if they want.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>