The help output for `thread backtrace` specifies that you can pass -1 to
`--count` to display all the frames.

```
-c <count> ( --count <count> )
            How many frames to display (-1 for all)
```

However, that doesn't work:

```
(lldb) thread backtrace --count -1
error: invalid integer value for option 'c'
```

The problem is that we store the option value as an unsigned and the
code to parse the string correctly rejects it. There's two ways to fix
this:

1. Make `m_count` a signed value so that it accepts negative values and
appease the parser. The function that prints the frames takes an
unsigned so a negative value will just become a really large positive
value, which is what the current implementation relies on.
2. Keep `m_count` unsigned and instead use 0 the magic value to show all
frames. I don't really see a point in not showing any frames at all,
plus that's already broken (`error: error displaying backtrace for
thread: "0x0001"`).

This patch implements (2) and at the same time improve the error
reporting so that we print the invalid value when we cannot parse it.

rdar://123881767