Fix a problem with select and high fds.

If on systems where the maximum number of fds may be dynamically
configured to a value of FD_MAXSIZE or higher and the RNG is first
used after more than FD_SETSIZE-1 descriptors are in use, we disable
the progress messages from the RNG. A better solution would be too
use poll but that requires more tests.

The same problem exists in rndunix.c - however this rng is only used
on old Unices and I assume that they don't feature dynamically
configured maximum fd sizes.