Re-establish backwards compatibility with abcl-1.4.0

Remove #+abcl conditional from SWANK-BACKEND on DEFIMPLEMENTATION in
preparation for re-merge with upstream source.  Unfortunately hooking
into the SWANK-BACKEND seems to be the only way to get this to work.

Use probes for the existence of functionality to load the
abcl-introspect contrib.

Fallback to local implementation of GETPID if not available in
implementation.