sock: refactore tcp keepalive setting
- Leave default number of keepalive probes rather than hardcoded value - QNX and OSX use TCP_KEEPALIVE instead of TCP_KEEPIDLE - QNX doesn't have TCP_KEEPINTVL
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- Leave default number of keepalive probes rather than hardcoded value - QNX and OSX use TCP_KEEPALIVE instead of TCP_KEEPIDLE - QNX doesn't have TCP_KEEPINTVL