Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

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.

handle something as convoluted as

  ftp://joe:secret@insecure.org:8081/hidden/path/to?what=is#this