[cmyth] Handle findday of recording rule

'findday' is {0 = Saturday, 1 = Sunday, …, 6 = Friday}. Compute value using ODBC standard (1 = Sunday, 2 = Monday, …, 7 = Saturday) modulo 7.
[mythtv-cmyth] cosmetics

Also comments cryptic code: Numeric hash value is used to make constant recording index even after cache updates. PVR use it to handle timer changes.