Clean-up in add_field.
Clean-up in add_field.

 * Rename $expected_fields to $input_field_count
 * Compile split() regexes once
 * Prepend new field in position 1 when -f value is negative and the abs.
   value is too great.

google/crush-tools,google/crush-tools,lexsf/crush-tools,google/crush-tools,lexsf/crush-tools,lexsf/crush-tools,google/crush-tools,lexsf/crush-tools,lexsf/crush-tools,google/crush-tools,google/crush-tools,google/crush-tools,lexsf/crush-tools,lexsf/crush-tools