Adding route() made me review dap_enable() more closely and it wasn't
doing what I intended to do before, it was ignoring I2S coming back from
Teensy, DAP was sourced from ADC as was I2S_DOUT and DAC was sourced
from DAP; Teensy received valid data from ADC but data from Teensy was
just discarded if user selected it.

unsigned short .adc_hpf(uint8_t bypass, uint8_t freeze);
unsigned short .adc_hpf(uint8_t bypass);
bypass & freeze as defined on page 34 of the SGTL5000 datasheet.

added last of dap 'bits' to control_sgtl5000.* and include an example
for bass enhance, need to make one for surround stuff.

Added
AudioControlSGTL5000::dap_avc(..);
AudioControlSGTL5000::dap_avc_enable(..);

SGTL5000_Specific/dap_avc_agc example sketch

Also added 'public domain...' line to all my examples.