[settings] Remove "uristring" setting type

Commit b5f5f73 ("[cmdline] Expand settings within each command-line
token individually") effectively rendered the "uristring" setting type
obsolete, since strings containing whitespace no longer break the
command line parser.  The concept of the "uristring" type is not well
defined, since URI escaping rules depend on which portion of a URI is
being escaped.

Remove the "uristring" type, converting it into an alias for the
"string" setting type so as to avoid breaking existing scripts.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[params] Use reference counters for form parameter lists

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[uri] Refactor URI parsing and formatting

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>
[image] Ensure every image has a fully resolved URI

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