Add dyndebug flags backtrace_when_lost, sleep_when_lost

Add dyndebug flags backtrace_when_lost, sleep_when_lost

Available when :SB-QSHOW is enabled, these settings can be enabled
through the SBCL_DYNDEBUG environment variable to customize SBCL's
behaviour prior to entry to ldb.

With backtrace_when_lost, lossage shows a backtrace before
continuing with the lossage handler as usual.  (Thanks to Dmitry
Kalyanov for this idea, here committed as a dyndebug option.)

With sleep_when_lost activated, the monitor is preempted, and SBCL
will instead cease to do anything except for a OS-level sleep call.
This behaviour has proven useful in preserving a failing thread's
current state until an external debugger can be attached.