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- currently implemented for darwin and linux

FreeBSD
Linux
Other platforms will have it undefined for now.

htop fails to compile with:

```text
SignalsPanel.c:32:49: error: use of undeclared identifier 'Platform_signals'
      Panel_set(this, i, (Object*) ListItem_new(Platform_signals[i].name, Platform_signals[i].number));
                                                ^
1 error generated.
*** Error code 1
```

Implementations for Linux (tested) and FreeBSD (still untested, thanks to @etosan for providing the table).
Darwin and OpenBSD(ping @mmcco) builds should be broken now, pending their own tables.

This will produce too much replicated code.
I think I'll use a lighter abstraction in things like this.