1.0.4.59: small signal handling improvements

1.0.4.59: small signal handling improvements

 * Grovel SIG_DFL and SIG_IGN.

 * Grovel "rare" signals based on the definedness of the signal, not
   LISP_FEATURE_FOO.

 * Make SIGINT signal an INTERACTIVE-INTERRUPT, which should make
   sense on Windows too, if we ever get the ConsoleCtrlHandler
   working.

 * Make SIGTRAP signal an BREAKPOINT-ERROR, and make SYSTEM-CONDITION
   a superclass of both it, INTERACTIVE-INTERRUPT and
   MEMORY-FAULT-ERROR.

 * Inhibit GC while destroying the thread mutex lutex to avoid races
   with the GC.

 ...and add missing NEWS entries due to your truly.