Rename SendResponse to SendDirectStackResponse in ocstack.c

Rename SendResponse to SendDirectStackResponse in ocstack.c

SendResponse is only used to send out ACKs, RESETs and error conditions
caught by the stack. The CA layer and the C++ SDK have SendResponse.
Renamed the function in ocstack.c to make the intent clearer.

Change-Id: I00a089273fd536d6dd30ac5f9e87ea0dc9fbb8b7
Signed-off-by: Mandeep Shetty <mandeep.shetty@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/833
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Doug Hudson <douglas.hudson@intel.com>
Reviewed-by: Joseph Morrow <joseph.l.morrow@intel.com>
Reviewed-by: Erich Keane <erich.keane@intel.com>