only queried when the /dev/[u]random devices did not return enough
entropy. Only the amount of entropy missing to reach the required minimum
is queried, as EGD may be drained.
Queried locations are: /etc/entropy, /var/run/egd-pool
them for a short period of time (actually, poll them with select(),
then read() whatever is there), which is about 10ms (hard-coded value)
each.
Separate Windows and Unixly code, and start on a VMS variant that
currently just returns 0.
