fix fd leak in test-concurrent

Opening a debug log for every thread at every iteration of test-concurrent
causes it to quickly run out of fd's because this fd is leaked. Fix this
by adding a new api: lxc_log_close().

As Caglar noted, the log handling is in general a bit "interesting" because
a logfile can be opened through the per-container api
c->set_config_item("lxc.logfile") but lxc_log_fd is now per-thread data. It
just so happens in test-concurrent that there is a 1:1 mapping of threads
to logfiles.

Split out getting debug logs from quiet since I think they are useful
separately. If debug is specified, get a log of any mode, not just during
start.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>