Don't use the "select" system call, use poll or epoll

[llothar]

In "ftplib::socket_wait" select is used which contains the unfixable problem of memory corruption and memory leaks on any linux system that uses more then 1024 file descriptions (which hopefully all systems will become one day as this limit is a laughable low amount from 1970 days of Unix)

Only poll and epoll are useable in modern systems.