Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

"override" implies "virtual".

Replaces map_to_relative_path() from Mapper.cxx.

libsmbclient is not thread-safe nor reentrant.  We must protect all
function calls with a global mutex, unfortunately.