This separates out the Linux socket TCP keepalive timeout interval
from Sofia's internal mechanisms.  Earlier we tied these together.  In
retrospect this seems improper.

These two values can now be set separately.

You might, for example, want to keep the Sofia internal mechanism
disabled completely while enabling the platform-based mechanism if
your platform supports it.

We also here reform the default value of the socket TCP keepalive
parameter to 30 seconds.

This is what commit 1bf17857 should
have been.

FS-6104