imtcp: add options to configure keep-alive params

New options:        (legacy counterparts)
keepalive.time      $inputtcpserverkeepalive_time
keepalive.probes    $inputtcpserverkeepalive_probes
keepalive.interval  $inputtcpserverkeepalive_interval

The options are set separately for each instance.

This patch bumps the interface version of the nsd,
netstrm and tcpsrv modules.
Merge pull request #145 from rsyslog/master-th-imtcp-keepalive-params

imtcp: add options to configure keep-alive params