This patch is the final refactoring work to have client library
for sbus communication, and port current implementaion about
execute command from runtime to SBusClient.
This patch makes execute command rely on the same communication
as the other commands, which happens on SERVICE_OUT_FD, and remove
TASK_ID_OUT_FD which is used only for returning task id.
This allows us to avoid special handling for execute command.
Change-Id: I00dc7b3004512624f9aaf120420f910e451cac89
826be15b6a