Fuller trigger & strobe support

Polarity of the trigger can be set, and strobe on pin 1 is supported.  Extension to pins 2 and 3 would be easy, but it's rarely used.
My editor shouldn't be doing this.  The pre-existing whitespace was a bit of mess.  Judging by alignment it looked like it had been edited last by someone expecting tabstops at 4 spaces, so I continued with that, but removed spaces at end-of-lines, and converted spaces before tabs to the requisite number of tabs to keep alignment.  I can instead leave out the whitespace canonization commit if you prefer.