MINOR: limits: keep a copy of the rough estimate of needed FDs in global struct

MINOR: limits: keep a copy of the rough estimate of needed FDs in global struct

It's always a pain to guess the number of FDs that can be needed by
listeners, checks, threads, pollers etc. We have this estimate in
global.maxsock before calling set_global_maxconn(), but we lose it
the line after. Let's copy it into global.est_fd_usage and keep it.
This will be helpful to try to provide more accurate suggestions for
maxconn.