Excellon format is clear that only two values
are valid for header_number_format, and
which value is used depends on the units:

• Inches ==> FMT_00_0000
• Metric ==> FMT_000_000

Therefore, do not set to FMT_USER on those
occasions, but to the allowed format type.

Also added many more comments to
drill_parse_header_is_metric(), explaining
the implied logic.

Fixed by #210.