This fixes "gave up waiting for init of module libcrc32c." which
happened at boot time due to multiple parallel module loads.

The problem was a deadlock: we wait for a module to finish
initializing, but we keep the module_lock mutex so it can't complete.
In particular, this could reasonably happen if a module does a
request_module() in its initialization routine.

So we change use_module() to return an errno rather than a bool, and if
it's -EBUSY we drop the lock and wait in the caller, then reaquire the
lock.

Reported-by: Brandon Philips <brandon@ifup.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Tested-by: Brandon Philips <brandon@ifup.org>