Make section targeting constants global for use in other modules

Section targeting is a great feature, but it should also define the ignore flag as a constant to be available to other modules for theming. I also suggest moving these section defines to the beginning of the module file, like this:

define(ADSENSE_START, "<!-- google_ad_section_start -->\n");
define(ADSENSE_IGNORE, "<!-- google_ad_section_start(weight=ignore) -->\n");
define(ADSENSE_END,   "<!-- google_ad_section_end -->\n");