app/testpmd: avoid exit without terminal restore

app/testpmd: avoid exit without terminal restore

In interactive mode, if testpmd exit by calling rte_exit without
restore terminal attributes, terminal will not echo keyboard input.

register a function with atexit() in prompt(), when exit() in
rte_exit() is called, the registered function restores terminal
attributes.

Fixes: 5a8fb55c48ab ("app/testpmd: support unidirectional configuration")
Cc: stable@dpdk.org
Signed-off-by: Dapeng Yu <dapengx.yu@intel.com>
Acked-by: Xiaoyun Li <xiaoyun.li@intel.com>