conf: Add description of AllowedTetheringTechnologies
main: Add support for AllowedTetheringTechnologies configuration variable
device: Add support functions needed by ethernet tethering
rtnl: Interface was added to technology too early

The __connman_technology_add_interface() needs to be called
after we have created the technology which is done by rtnl->newlink()
callback. If this is done the old way, the ethernet devices
will not get their interfaces set properly.
ethernet: Add tethering support
conf: Add description of PersistentTetheringMode
main: Add support for PersistentTetheringMode configuration variable
technology: Auto start tethering if enabled and configured

If the user has enabled persistent tethering mode in main.conf, then
try to activate tethering when technology is re-enabled, or we are
returning from offline mode, or after the device has rebooted.
dnsproxy: Return proper return code if listener_table is missing

The listener_table can be NULL if we have started connman with -r
option. Because we return < 0 value to caller in this case,
the caller, e.g. the tethering code, has a possibility to change 
its functionality, e.g. the tethering code can fall back to some
other DNS servers.
tethering: Use fallback nameservers if dnsproxy is not in use

If user has disabled dnsproxy with -r option, then fallback to
user specified nameservers instead of using the Google one.
service: Replace connman_uint8_t with uint8_t
network: Replace connman_uint8_t with uint8_t
include: Remove definition for connman_uint8_t
network: Replace connman_uint16_t with uint16_t
include: Remove definition for connman_uint16_t
TODO: Update type definitions cleanup task
gdhcp: Add debug print for transaction id

Print transaction id of sent and received messages so that it is
easier to match the packets.
gdhcp: Add helper functions for message retransmission
gdhcp: Do not change the transaction id if retransmitting

As requested in RFC 3315 chapter 15.1, a client MUST leave
the transaction id unchanged in retransmissions of a message.
dhcpv6: Retransmit a message with same transaction id

Make sure we do not create a new transaction id for a message
when it needs to be retransmitted.