daemon: Make TCP keep-alive time-out configurable

The option "keepalive_timeout" now defines a keep-alive time-out in seconds for
connections. The TCP-CL connection sends its keep-alive packets in this interval.
android-daemon: Increase keep-alive timeout interval

The new option "keepalive_timeout" allows to define a larger interval for TCP
keep-alive packets. We increase this value to keep the device longer in sleep mode
and save some energy.
Merge pull request #67 from morgenroth/Increase-keepalive-timeout

Increase keep-alive time-out