Fix handling of SIGINT by router executable

[Yadunund]

Aim to address #78

According to the manual page(), system will return -1 on error.

However right now, killing the process via SIGINT (CTRL+C) returns with error code 2. Investigating...

[2024-01-23T14:48:43Z INFO  zenohd] Starting required plugin "rest"
[2024-01-23T14:48:43Z INFO  zenohd] Successfully started plugin rest from "/usr/lib/libzenoh_plugin_rest.so"
[2024-01-23T14:48:43Z INFO  zenohd] Finished loading plugins
^C
Thread 1 "zenohd" received signal SIGINT, Interrupt.
syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
38	../sysdeps/unix/sysv/linux/x86_64/syscall.S: No such file or directory.
(gdb) bt
#0  syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
#1  0x00005555557a6762 in parking::Inner::park ()
#2  0x0000555555696c43 in zenohd::main ()
#3  0x000055555561eaf3 in std::sys_common::backtrace::__rust_begin_short_backtrace ()
#4  0x00005555556b23a7 in main ()
(gdb) print $eax
$1 = -512

[clalancette]

When running this on rolling, I get:

^CFailed to run zenoh router: zenohd -c /home/clalancette/zenoh_ws/install/rmw_zenoh_cpp/share/rmw_zenoh_cpp/config/DEFAULT_RMW_ZENOH_ROUTER_CONFIG.json5
[ros2run]: Process exited with failure 2

With this PR in place, I get:

^CZenoh router exited abnormally with error code [2]

So it is overall an improvement.

I'll just point out that almost certainly we are going to have to rethink this completely when we start testing this on Windows.