Support long file names on Windows; more CRT function avoidance

Support long file names on Windows; more CRT function avoidance

  - Use native namestrings of the form \\?\ and \\?\UNC\ as required.

  - UNC pathnames now represented using the keyword :UNC in the device.
    This aspect of the implementation is user-visible, but considered
    subject to change.

  - Update a few final holdouts among the file system routines which
    were still using CRT functions and replace them with native versions:
    FILE-LENGTH, QUERY-FILE-SYSTEM, DELETE-FILE, DELETE-DIRECTORY.

  - Sneakily include a getenv change, technically unrelated.

Thanks to Anton Kovalenko.