Move the RC_MODES enum to the public header

This allows users to know what values to set for the iRCMode
parameter.
Make the iRCMode field use the RC_MODES type instead of plain int

This makes it even clearer for users about how to set this field.
Merge pull request #561 from mstorsjo/use-rc-enum

Make the RC_MODES enum public, use it for the iRCMode field