sbig: get sbig_ccd_get_max()

We will try focus by coarse constrast detection.
libsbig: document defualt ccd values
libsbig: sbig_ccd_get_exposure_status use enum

Instead of a ushort, return PAR_COMMAND_STATUS enum
cmd: add sbig-snap
libusb: change unsigned short to ushort
libsbig: sbig_ccd_set/clr_exposure_flags()
cmd: further work on sbig-snap
libsbig: fix bug in writepgm
libsbig: fix another bug in writepgm

Height and width were reversed in header
libsbig: writepgm add comments to file

Record some telemetry info in the PGM file, as comments.
cmd: add some command line options to sbig-snap
cmd: sbig-snap: add --dark option