client: Split client_open() and client_close().

client: Split client_open() and client_close().

For interactive para_client sessions, we set up the client
configuration from the values given at the command line and in the
config file just as we do for non-interactive mode. However, in
interactive mode we must be able to execute arbitrary many client
commands using the same configuration. client_open() sets up the
configuration from the command line arguments and client_close()
destroys the configuration, so both functions do more than what is
needed in interactive mode.

This patch splits client_open() into two functions
client_parse_config() and client_connect(). The old client_open()
remains with the same semantics as before, it just calls both of the
new functions().

In the same spirit, client_close() is split into client_disconnect(),
which closes the file descriptor and deallocates the ressources of
the current command but does not free the configuration.

This allows to re-use the client configuration multiple times for
interactive sessions.