Bugfix: 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