1.0.37.6: Add SB-SYS:CANCEL-DEADLINE restart to DEADLINE-TIMEOUTs.

1.0.37.6: Add SB-SYS:CANCEL-DEADLINE restart to DEADLINE-TIMEOUTs.

  * Establish an SB-SYS:CANCEL-DEADLINE restart in SIGNAL-DEADLINE.

  * Add an SB-SYS:CANCEL-DEADLINE restart function.

  * Make SB-INT:READ-EVALUATED-FORM take an optional prompt. This
    function is commonly used to query the user for input in restarts.
    Use it in the SB-SYS:DEFER-DEADLINE restart in SIGNAL-DEADLINE.

  * Bind *DEADLINE-SECONDS* in SB-THREAD:MAKE-THREAD. Not binding it
    does not seem to have severe consequences, but that's not obvious
    so just bind both so humans won't waste brain cycles on
    it. SB-KERNEL:SUB-GC also binds both.

  * Add usage of WITH-TEST to tests/deadline.impure.lisp. Also add
    a test case for the new CANCEL-DEADLINE restart.