netbase: Make SOCKS5 negotiation interruptible

Avoids that SOCKS5 negotiation will hold up the shutdown process.

- Sockets can stay in non-blocking mode, no need to switch it on/off
  anymore
- Adds a timeout (20 seconds) on SOCK5 negotiation. This should be
  enough for even Tor to get a connection to a hidden service, and
  avoids blocking the opencon thread indefinitely on a hanging proxy.

Fixes #2954.