2.016.2: Enhancements for MCL pathname and logical-pathname users.

Major change for RMCL: location strings are now POSIX namestrings.
Major change for logical pathnames: they are now preserved by source-registry;
the price is that only logical pathnames for which merging the name and type
commutes with truename will be found by directory-files and subdirectories.
Minor change for GCL: dropped kluges for 2.6, which never was working.
Simplifications in internals of (implementation-identifier).
Removed *system-cache* and get-uid, which never were working right
(bad attempts at allowing common-lisp-controller compatibility,
but it only ever was but a giant gaping security hole in the waiting).
Simplified resolve-relative-location-component
(ugly because it was written before MERGE-PATHNAMES*).
Updated documentation about the file location DSL.