Allow INFO field name to have underscores

Lines 271 and 301 in vcfquery.c should do the same thing.

As noted previously [1], `%SOME_NAME` in the format string will work without this fix. However, `%INFO/SOME_NAME` will be parsed as `%INFO/SOME` without this fix.

[1]: https://github.com/samtools/htslib/pull/21#issuecomment-22306162