Break up the maxSocketsAllowed assignment.
Limit maxSocketsAllowed to FD_SETSIZE.

At high scan rates, there was nothing to prevent trying to set or clear
socket descriptors above FD_SETSIZE, which is usually around 1024. I got
a reliable assertion failures with the command
	nmap -sT -p- --min-rate 100000 scanme.nmap.org
The problem only affected -sT scans. A similar protection was added to
Nsock in r15808.