Support outputting strobes.

Note that one camera may output strobes on multiple pins, with
different configurations, per pin.  Unlike the trigger, there's
no need to select one pin.  For now GPIO pin 1 is the only one
supported.  strobe2 and strobe3 could be added if anyone ever needs
them, but it seems unlikely.
Refactor GPIO pin comparison into separate function.
Read camera's resulting strobe configuration.
Read camera's resulting trigger configuration.