dmb(3) error reporting

[utzoo!henry]

Contrary to the manual page, the store() routine of dbm(3) actually
does not return an error code at all.  This can be fixed by the
following mods to /usr/src/libdbm/dbm.c:

	113c113
	< 	return;
	---
	> 	return(0);
	118c118
	< 		return;
	---
	> 		return(-1);