1.0.5.9: experimental semi-synchronous deadlines

1.0.5.9: experimental semi-synchronous deadlines

 * WITH-DEADLINE provides an interface to a synchronous deadline/timeout
   facility that can interrupt execution only on blocking IO and when
   waiting on locks (latter Linux only for now.)

 * DECODE-DEADLINE provides an interface that implementors of blocking
   functions can use to hook into the deadline mechanism.

 * Add SB-IMPL::*ON-DANGEROUS-SELECT* for debugging: can be used to
   warn/ signal an error / obtain a backtrace when SBCL calls select
   without a timeout while interrupts are disabled.

 * Undocumented and unexported periodic polling functionality has been
   removed from SERVE-EVENT, but can be reinstated should it be
   desired.