src/master: minor comment clarification
src/master: fix FIXME (mystery constant without comment)
src/slave: can now address 3 slaves without lock-ups (MISO conflict)

Issue with master locking up when more than one slave connected was due
to all slaves trying to constantly drive the MISO line. Now Slaves' Data
Out (MISO) line is tri-state (Hi-Z) by default, until Master sends an
interrupt request to that specific Slave.

This commit appropriately modifies:
* spi_slave_init() in spi.c
* ISR (PCINT_vect, ISR_NOBLOCK) in master.c