With Visual Studio versions 2008 and older they ignore the full path to files and only check
the basename of the file to find a collision. Additionally, having duplicate basenames can break
other build tools like GYP.

This fixes https://github.com/libgit2/libgit2/issues/3356

Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).

Some brain damaged tolower() implementations appear to want to
take the locale into account, and this may require taking some
insanely aggressive lock on the locale and slowing down what should
be the most trivial of trivial calls for people who just want to
downcase ASCII.

Treat input bytes as unsigned before doing arithmetic on them,
lest we look at some non-ASCII byte (like a UTF-8 character) as a
negative value and perform the comparison incorrectly.

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.