home/limit switches on I2C
add I2C read-modify-write and transaction management (not tested yet)
I2C transaction queue init call
update I2C home pin assignments; disable twi queues until tested
insert neglected pointer deref in twi_check_queues; parentheses to suppress compiler warn
queued I2C read: initial test OK; prep for interrupt-driven read
checkpoint: I2C INT0-driven update appears to work
a bit of de-crufting
comment changes only
I2C queued write (e.g. spindle control) working; not thoroughly tested