MINOR: http-htx: Add functions to create HTX redirect message

MINOR: http-htx: Add functions to create HTX redirect message

http_parse_errorloc() may now be used to create an HTTP 302 or 303 redirect
message with a specific url passed as parameter. A parameter is used to known if
it is a 302 or a 303 redirect. A status code is passed as parameter. It must be
one of the supported HTTP error codes to be valid. Otherwise an error is
returned. It aims to be used to parse "errorloc" directives. It relies on
http_load_errormsg() to do most of the job, ie converting it in HTX.