Accessing GPIO pins via the sysfs interface is deprecated. When
PTT_TYPE is set to GPIOD there are two new configuration variables
available to set GPIOD up, PTT_GPIOD_CHIP and PTT_GPIOD_LINE. When
SQL_DET is set to GPIOD there are two new configuration variables
available to set GPIOD up, SQL_GPIOD_CHIP, SQL_GPIOD_LINE. If SvxLink is
built with libgpiod >= 1.5 SQL_GPIOD_BIAS will also be available to set
PULLUP/PULLDOWN for a GPIO input (squelch) pin.
Closes#538