patch to make backtrace behave better in case of infinite nested errors

[Attila Lendvai]

a pull request on github:

https://github.com/sbcl/sbcl/pull/3

Added an EMERGENCY-BEST-EFFORT keyword argument to PRINT-BACKTRACE,
disabled by default. The debugger calls PRINT-BACKTRACE with the
flag on only when it gets nested, so that e.g. errors from printing
the backtrace also land in a (nested) debugger, but only when
coming from the first level of the debugger.

Make ERROR-ERROR try to print more info on the way down into
nesting and use this new keyword.

[Attila Lendvai (attila-lendvai)]

i see some anomalies, so please ignore the patch until further notice.

[Attila Lendvai (attila-lendvai)]

patch rebased.

the problems i've seen were fixed by a5839f2c1df6f1bc228d79c41103618e30f0147c

Respect *DEBUG-PRINT-VARIABLE-ALIST* when printing args in PRINT-FRAME-CALL.