sbig-snap: add --image-type argument

sbig-snap --image-type=DF   # take dark frame (only)
sbig-snap --image-type=LF   # take light frame (only)
sbig-snap --image-type=AUTO (default)  # take DF + LF, subtract, write LF
libsbig: add sbfits_set_imagetype()
doc: update for IMAGETYP FITS header value
sbig-snap: set IMAGETYP in FITS header
sbig-snap: remove some dead code
sbig-focus: new

Add utility for focusing/alignment that takes non-persistent images
in a tight loop until the user types ctrl-C.

Fixes issue #1