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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.

Added preliminary attempt at jails support on FreeBSD - JID and JAIL (name) columns, somewhat more correct kernel "thread" detection.

Seems FreeBSD kernel can spawn both kernel processes (what is what htop currently sees) and kernel threads.
For now let's consider kernel processes kernel "threads".

* size_t nmemb (number of elements) first, then size_t size
* do not assume char is size 1 but use sizeof()
* allocate for char, not pointer to char (found by Michael McConville,
  fixes #261)

Closes #228.

`Process_new_fn` had been renamed to `Process_New` in
d880def0

Several functions and struct had changed in
b291fba0

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