Add support for MKCL

* lib/lisp/backend/slynk-mkcl.lisp: New file. Mostly the one from MLCL's contrib/
directory.
* slynk-loader.lisp (*sysdep-files*): Add slynk-mkcl and move
slynk-gray to the shared *slynk-files*.
* slynk.asd: Add MKCL and remove reader conditional from slynk-gray component.

(cherry picked from SLIME commit
102942bb6f9e60e07961a1844a762dcf7b336a77)

Conflicts:
	ChangeLog
	slynk-loader.lisp