Use new primitive `microcode-library-path', if present, to get a

Use new primitive `microcode-library-path', if present, to get a
sequence of library directory names.

Change `system-library-directory-pathname' to search through this
sequence; also change it to accept an argument which is the name of a
subdirectory to look for.

New procedure `system-library-pathname' searches for a given file in
one of the library directories.

`load-option' and `disk-restore' changed to use
`system-library-pathname'.