luci-lib-httpclient: revamp URL handling

 - Introduce a parse_url() helper which properly deals with literal IPv4
   and IPv6 host parts and returns the decomposed uri string as table
 - Properly format IPv6 literals in autogenerated Host headers
 - Omit default port numbers in Host headers and calculated redirect URLs.

Supersedes PR #2337.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>