Reduce overhead by not duplicating the string.

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.

New in clang 3.6.

A file called ".jpg" is not a JPEG file with an empty name; it is
merely a hidden file.

Replaces g_uri_parse_scheme().

Micro-optimization.

g_basename() is deprecated in GLib 2.32.  Instead, verify that the
suffix does not have a backslash, to catch Windows path names.

This buggy implementation failed to allow "..." sequences, because the
dot count was always zero.  The usefulness of allowing "..." (or more
dots) is debatable, but since it's a valid file name, we allow it.

Dots at the beginning of an URI segment are ok, as long as the special
names "." and ".." are not used.

If there's a slash in the uri_get_suffix() return value, then it's
malformed.  Return NULL in this case.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.