Synchronize Sofia and Linux TCP keepalive timeout units

Sofia keeps the TCP keepalive timeout in milliseconds, but Linux
expects the value in seconds.  Before this change, it's unlikely the
TCP_KEEPIDLE and TCP_KEEPINTVL calls were having much effect as we
would have been passing them a huge value.

FS-6104
Add additional debugging output to keepalive handling
Rework handling of Linux TCP keepalives in Sofia

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 a0e9639a1f38f26a5dc586d386ae044ba82e0137 should
have been.

FS-6104
Add mod_sofia support for setting socket TCP keepalive

Some OSes like linux provide a mechanism for sending TCP keepalive
pings.  Sofia provides its own mechanism for doing this in userspace.
We shouldn't conflate these two mechanisms.

FS-6104