* lib/sgroupio.c: Avoid assignments in comparisons.

	* lib/sgroupio.c: Avoid assignments in comparisons.
	* lib/sgroupio.c: Add brackets.
	* lib/sgroupio.c: Avoid implicit conversion of pointers / integers to booleans.
	* lib/sgroupio.c: The changed, isopen, locked, and readonly fields of the db are booleans.
	* lib/sgroupio.h, lib/sgroupio.c: sgr_file_present returns a bool.
	* lib/sgroupio.h: Add protection against multiple inclusions.