transport: allow configuration of source IP address

add the member bindaddr to tr_ssh_config and tr_tcp_config to allow
configuration of the source interface which is used to establish the
connection to an RTR-server

Signed-off-by: Fabian Holler <mail@fholler.de>
- merge commits to one, extend commit message
- initialize PTRs with with NULL instead of 0
doxygen: remove ssh_pubkey assignment from struct tr_ssh_config

The ssh_pubkey member was removed in commit d50a2b8