random-util: rename RANDOM_DONT_DRAIN → RANDOM_MAY_FAIL

The old flag name was a bit of a misnomer, as /dev/urandom cannot be
"drained". Once it's initialized it's initialized and then is good
forever. (Only /dev/random has a concept of 'draining', but we never use
that, as it's an obsolete interface).

The flag is still useful though, since it allows us to suppress accesses
to the random pool while it is not initialized, as that trips up the
kernel and it logs about any such attempts, which we really don't want.