URL-parser: for file://[host]/ URLs, the [host] must be localhost

Previously, the [host] part was just ignored which made libcurl accept
strange URLs misleading users. like "file://etc/passwd" which might've
looked like it refers to "/etc/passwd" but is just "/passwd" since the
"etc" is an ignored host name.

Reported-by: Mike Crowe
Assisted-by: Kamil Dudka