Merge the functionality of parse_next_server_and_filename() and
tftp_uri() into a single pxe_uri(), which takes a server address
(IPv4/IPv6/none) and a filename, and produces a URI using the rule:

 - if the filename is a hierarchical absolute URI (i.e. includes a
   scheme such as "http://" or "tftp://") then use that URI and ignore
   the server address,

 - otherwise, if the server address is recognised (according to
   sa_family) then construct a TFTP URI based on the server address,
   port, and filename

 - otherwise fail.

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