Handle Linux TCP keepalives better in Sofia

Sofia accepts a value for the TCP keepalive timeout interval via
TPTAG_KEEPALIVE, however it fails to use this value for the Linux
keepalive socket options TCP_KEEPIDLE and TCP_KEEPINTVL.  In fact, on
Linux it enables the sending of TCP keepalives even if tpp_keepalive
is set to zero which would disable Sofia's internal keepalive
mechanisms.  Sofia then uses a hard coded value of 30 seconds for
these keepalive intervals which affects battery life on mobile
devices.

With this commit we harmonize the sending of TCP keepalives on Linux
with other platforms by using the value from TPTAG_KEEPALIVE and not
enabling the sending of TCP keepalives at all if the value of the
parameter is zero.

FS-6104 --resolve