Add support for parsing of URIs containing literal IPv6 addresses
(e.g. "http://[fe80::69ff:fe50:5845%25net0]/boot.ipxe").

Duplicate URIs by directly copying the relevant fields, rather than by
formatting and reparsing a URI string.  This relaxes the requirements
on the URI formatting code and allows it to focus on generating
human-readable URIs (e.g. by not escaping ':' characters within
literal IPv6 addresses).  As a side-effect, this allows relative URIs
containing parameter lists (e.g. "../boot.php##params") to function
as expected.

Add validity check for FTP paths to ensure that only printable
characters are accepted (since FTP is a human-readable line-based
protocol with no support for character escaping).

Construct TFTP next-server+filename URIs directly, rather than parsing
a constructed "tftp://..." string,

Add self-tests for URI functions.

Signed-off-by: Michael Brown <mcb30@ipxe.org>