[uri] Avoid interpreting DOS-style path names as opaque URIs

[uri] Avoid interpreting DOS-style path names as opaque URIs

A DOS-style full path name such as "C:\Program Files\tftpboot\nbp.0"
satisfies the syntax requirements for a URI with a scheme of "C" and
an opaque portion of "\Program Files\tftpboot\nbp.0".

Add a check in parse_uri() to ignore schemes that are apparently only
a single character long; this avoids interpreting DOS-style paths in
this way, and shouldn't affect any practical URI scheme.