Fix case-sensitivity bug in zone filename token-parsing

Fix case-sensitivity bug in zone filename token-parsing

The setfilename() function uses case-insensitive strcasestr() when
matching the possible tokens, but then one of the token parsers
uses case-sensitive INSIST checks which can assert when, for example,
matching '%X' and INSIST only accepts '%x'.

The case-insensitivity is documented, which means it's the parser
that needs to be fixed, not the matcher.

Convert the character to lowercase before checking the token's
validity.