fix: usr: '{&dns}' is as valid as '{?dns}' in a SVCB's dohpath

dig fails to parse a valid (as far as I can tell, and accepted by kdig and Wireshark) SVCB record with a dohpath URI template containing a {&dns}, like dohpath=/some/path?key=value{&dns}". If the URI template contains a {?dns} instead dig is happy, but my understanding of rfc9461 and section 1.2. "Levels and Expression Types" of rfc6570 is that {&dns} is valid. See for example section 1.2. "Levels and Expression Types" of rfc6570.

Note that Peter van Dijk suggested that {dns} and {dns,someothervar} might be valid forms as well, so my patch might be too restrictive, although it's anyone's guess how DoH clients would handle complex templates.

Closes #4922