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/impossible to follow
the common <=C90 declaration-after-statement style.
transport: copy transport config in tr_tcp_open(..)

Copy the tr_tcp_config that is passed to tr_tcp_open(..) instead of
copying only the pointer.

The library user doesn't have to care anymore that the tr_tcp_config is
allocated on the heap and is valid while the transport is used.
transport: copy transport config in tr_ssh_open(..)

Copy the tr_ssh_config that is passed to tr_ssh_open(..) instead of
copying only the pointer.

The library user doesn't have to care anymore that the tr_ssh_config is
allocated on the heap and is valid while the transport is used.
tools: increase rtrclient polling_period to 30 seconds

Previously the RTR server was polled every second for updates.
For most purposes it's sufficient to ask the RTR server every 30sec for
updates.
transport: Check if malloc(..) failed in tr_tcp_open(..)

return TR_ERROR if malloc(..) returns NULL
transport: Check if malloc(..) failed in tr_ssh_open(..)

return TR_ERROR if malloc(..) returns NULL