canPortBeUsed(): Upgrade to better, native implementation from port4me'

[HenrikBengtsson]

The internal canPortBeUsed() function rely solely on serverSocket(port) to infer if a TCP port is available or not. That's not always sufficient. In port4me, we added a more robust check(*)

• https://github.com/HenrikBengtsson/port4me/blob/develop/r/R/is_tcp_port_available.R
• https://github.com/HenrikBengtsson/port4me/blob/develop/r/src/test_tcp_port.c

(*) Adopted from https://github.com/ropensci/ssh/blob/master/src/tunnel.c. FWIW, there's now also https://github.com/jeroen/curl/blob/master/src/findport.c, which stems from the former.