unix: support full TCP keep-alive on Solaris

Solaris claimed it supported the TCP-Alives mechanism,
but TCP_KEEPIDLE, TCP_KEEPINTVL, and TCP_KEEPCNT were not
available on Solaris until the latest version 11.4.
Therefore, we need to simulate the TCP-Alives mechanism on
other platforms via TCP_KEEPALIVE_THRESHOLD + TCP_KEEPALIVE_ABORT_THRESHOLD.