xbps_api.h: do not rely on sys/cdefs.h, fix up some comments.
xbps_api.h: do not rely on sys/cdefs.h, fix up some comments.

ebfe/xbps,stpx/xbps,stpx/xbps,ebfe/xbps,datenwolf/xbps,datenwolf/xbps,datenwolf/xbps,stpx/xbps,ebfe/xbps