Adapt GDB integration to newest patches

Adapt GDB integration to newest patches

* libguile/libguile-2.2-gdb.scm (vm-frame-function-name): Don't default
  to the address, as we will have better identifying info via the file
  name.
  (vm-frame-source): New helper.
  (compile-time-cond): For some reason "else" matching wasn't working;
  punt and use expressions.
  (snarfy-frame-decorator): Rename from decorator, and adapt to new
  version of Guile frame filter patch.
  (vm-frame-filter): Adapt to frame filter changes, and fill in source
  info.