gpioctl: Print interrupts capabilities

GPIO drivers who supports interrupts report them in the caps
(obtain via the getcaps method) but gpioctl doesn't know
how to interpret this and print "UNKNOWN" for each one of them.
Even if we don't have userland gpio interrupts support for now
let gpioctl print the supported caps.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24133