Fix F_SET / F_ISSET confusion

In WiredTiger we have macros for dealing with bitwise flags called F_SET (to set a flag) and F_ISSET (to check whether a flag is set). It is easy to confuse these two, and accidentally set a flag when intending to check for it.

Make F_SET (and F_CLR while we're in the neighborhood) into void expressions to avoid this class of bug.