src/master: remove old code from slave_is_available()
src/master: remove reference to old single OK line in code from spi.h
sch/master: updated schematic with 3 separate OUT OK lines
src/master: enable and fix reading address/settings from panel

Uncommented 2-year-old code and fixed small logic glitch that was
introduced by fiddling with spi_master_transmit().

Updated pin definitions.

Improved comments on why some actions are needed.
src/slave: don't fire channels if firing angle is 0

If ZC interrupt pulse duration is not set properly (is too long),
fire_channels() would be executed when zc.angle counter reaches zero.
src/master: fix bugs in ADC code that prevented 3-slave operation

Don't disable digital inputs on PORTC (they are used by other subsystems).

Rewrote channel selection definitions.

Add explanation comments, update some previous comments to be more
specific.

ADC is not yet enabled, there seems to be some kind of a glitch. Probably
ADC could be read once every 'for(12 channels)' iteration, but even just
once at start-up would be sufficient. However, there are (currently) two
ADC channels to be read, and only the last one's value can be truly
received. Perhaps this is due to the way ADC channels are currently
multiplexed - without stopping the free-running ADC).