Going to sleep intentionally while holding a mutex is an application
issue. When user consistency checks are enabled (XENO_OPT_DEBUG_USER),
any attempt to do so will trigger a SIGDEBUG signal to the offending
thread, conveying the SIGDEBUG_RESCNT_SLEEP code.

This change implicitly disables fast locking optimizations for
user-space threads when XENO_OPT_DEBUG_USER is enabled. Since the
debugging code present or future may already add some overhead, this
seems acceptable anyway.