Turn invalid max-client-connections values into fatal errors

Turn invalid max-client-connections values into fatal errors

Use FD_SETSIZE/2 as upper limit because Privoxy needs two
sockets to serve client connections that need forwarding.

Ignore the fact that the first three file descriptors
are usually set to /dev/null, one is used for logging
and yet another file descriptor is required to load
config files.

Make an exception for _WIN32 as its select() is documented
to handle file descriptors above FD_SETSIZE just fine.