126.96.36.199: workaround for bug #412 & undefined variable cleanup
* When converting dead references to global variables, convert using
SYMBOL-VALUE. Results in the same code, but prevents dead unbound
variable references from being flushed.
* Collect XREF information for constant SYMBOL-VALUE cases.
* Fix one dead reference to *LEXENV* in SBCL codebase.
* Fix other undefined variables in contribs.