Disallow:
 1. paths with trailing dot
 2. paths with trailing space
 3. paths with trailing colon
 4. paths that are 8.3 short names of .git folders ("GIT~1")
 5. paths that are reserved path names (COM1, LPT1, etc).
 6. paths with reserved DOS characters (colons, asterisks, etc)

These paths would (without \\?\ syntax) be elided to other paths - for
example, ".git." would be written as ".git".  As a result, writing these
paths literally (using \\?\ syntax) makes them hard to operate with from
the shell, Windows Explorer or other tools.  Disallow these.

When turning UTF-8 paths into UCS-2 paths for Windows, always use
the \\?\-prefixed paths.  Because this bypasses the system's
path canonicalization, handle the canonicalization functions ourselves.

We must:
 1. always use a backslash as a directory separator
 2. only use a single backslash between directories
 3. not rely on the system to translate "." and ".." in paths
 4. remove trailing backslashes, except at the drive root (C:\)

A reparse point that is an IO_REPARSE_TAG_MOUNT_POINT could be
a junction or an actual filesystem mount point.  (Who knew?)
If it's the latter, its reparse point will report the actual
volume information \??\Volume{GUID}\ and we should not attempt
to dereference that further, instead readlink should report
EINVAL since it's not a symlink / junction and its original
path was canonical.

Yes, really.

* Removes mingw-compat.h
* Cleans up separation of compiler/platform idiosyncrasies
* Unifies mingw/msvc stat structures and functions
* (Tries to) hide more compiler specific implementation details (even in our internal API)