Parse the URI template and check for a dns variable

Parse the URI template and check for a dns variable

The 'dns' variable in dohpath can be in various forms ({?dns},
{dns}, {&dns} etc.).  To check for a valid dohpath it ends up
being simpler to just parse the URI template rather than looking
for all the various forms if substring.

(cherry picked from commit af54ef9f5d703ed61c140f3e32e6fab5d88923a1)