create_conn: prune dead connections

Bringing back the old functionality that was mistakenly removed when the
connection cache was remade. When creating a new connection, all the
existing ones are checked and those that are known to be dead get
disconnected for real and removed from the connection cache. It helps
the cache from holding on to very many stale connections and aids in
keeping down the number of system sockets in wait states.

Help-by: Jonatan Vela <jonatan.vela@ergon.ch>

Bug: http://curl.haxx.se/mail/lib-2014-06/0189.html