0.8.17.19: Late resolution for foreign symbols &co

0.8.17.19: Late resolution for foreign symbols &co
            * If a foreign symbol is unknown, use an address on a
               protected page for it: all accesses there are trapped,
               and signal UNDEFINED-ALIEN-ERROR. (Currently the error
               doesn't reveal the name of the alien that was
               accessed.)
            * Make GET-DYNAMIC-FOREIGN-SYMBOL-ADDRESS keep track of
               both undefined aliens and dynamic foreign symbols in
               general.
            * Fix linkage-table reinitialization bug on threaded
               platforms, reported by Sean Ross.
            * Better restarts for realoding shared objects, and
               CONTINUE restarts for *save-hooks* and *init-hooks*.
            * Accurate alien warning for SAVE-LISP-AND-DIE on
               non-linkage-table platforms.

 HEALTH WARNING: Tested on x86/FreeBSD only, but "should be fine".