Limit memory used for select() statistics. It was allocating space for

Limit memory used for select() statistics. It was allocating space for
up to SQUID_MAXFD active filedescriptors per select(), but we are
only interested in a much smaller range.

This statistics could use quite a bit of memory when SQUID_MAXFD is large
(think I measured something like 8MB for 16K filedescriptors)