Actively manage number of open file descriptors

[droe]

With WebSockets and HTTP/2 there will be more persistent connections. Therefore actively managing the number of open file descriptors is critical. So far we relied on appropriate rlimits set by the user outside of SSLsplit.

Tasks:

• Look into how the number of open file descriptors scales with the number of concurrent connections in the parent and the child process, respectively; depends also on log settings used
• Add a «maximum number of concurrent connections» config option and calculate and configure appropriate RLIMIT_NOFILE levels based on the current configuration and a safety margin
• Refuse connections gracefully when going over the maximum number of concurrent connections limit; this should be implemented in pxy_thrmgr_attach() by summing up the ->load of each thread, and if that's over the connection limit, decline the connection by returning an error (-1)

This approach is more natural to configure than explicit rlimits and does not require requesting the count of open file descriptors at each new connection, with its reliability, portability and performance issues.