Key net.tcp.service[ntp] does not work with UDP

Generally NTP uses only UDP as transport. TCP almost not used at all. So, key "net.tcp.service[ntp]" which tries to use TCP as transport is useless.

I see a 3 different way to solve:
1. Remove support of NTP as parameter to key net.tcp.service[] to not mislead users anymore.
2. Make a note to documentation about NTP ant TCP and their useless.
3. Introduce new key net.udp.service[ntp] and remove NTP from net.tcp.service[].