transport: remove const modifier from tr_{ssh,tcp}_config members

Remove the const modifier from tr_tcp_config and tr_ssh_config elements.
The const modifier for the elements made it hard to follow
the common declaration-after-statement style.
It also stands in the way when the variable was created and the
assignment should be done in a different scope, e.g. in the branch of an
if-statement
rtrclient: fix segfault, access to out of scope tr_config members

A segfault happened in rtrclient shortly after start.
The ssh_config and tcp_config variables were
defined in the block scope of an if-statement. In this block the memory
address of the config structs were passed to the transport init
functions.
When the config were accessed in a later state of the program again, the
scope of the config variables was expired.

(Thanks to David Mandelberg and Michael Mester for reporting and
locating the cause of the segfault)