doc: *invoke-debugger-hook* -> *debugger-hook*

It looks like *debugger-hook* works just as well as
sb-ext:*invoke-debugger-hook* for controlling application termination,
so I this patch update the documentation to just mention *debugger-hook*
which has the benefit of being portable CL.
remove let-binding of default pathname in ccl

This fixes a bug in native-namestring for CCL reported by krrrcks.
Merge pull request #11 from eschulte/master

doc: *invoke-debugger-hook* -> *debugger-hook*