Windows "\\\\?\\", "\\\\?\\UNC\\", and "\\\\.\\" path meta characters

[Beman Dawes]

Windows treats "

?
", "

?
UNC
", and "

.
" at the start of a path as meta characters signaling special handling of the remainder of the path. So, for example, "

?
" should be treated as an empty path!

See "Naming Files, Paths, and Namespaces", ​http://msdn.microsoft.com/en-us/library/aa365247%28v=vs.85%29.aspx

Ticket 5405 supplied a patch that allows class path to handle a portion of what this implies, but does not deal with the general problem.

A few test cases have been added to path_test.cpp, but they are currently commented out.

[Beman Dawes]

The WikiFormatting messed up the meta characters. Should be

"\\\\?\\", "\\\\?\\UNC\\", and "\\\\.\\"

[Beman Dawes]

(In [71157]) Fix #5405. Note that this is only a partial fix. The issue is much larger; see ticket 5448 for details.