Fix https://github.com/mate-desktop/engrampa/issues/515

Fix https://github.com/mate-desktop/engrampa/issues/510

Fix https://github.com/mate-desktop/engrampa/issues/501

I had been relying on `unrar-free` being available as `unrar`, but that
seems to be a Debian thing, so accept the `unrar-free` itself as an
alternative.

The code has been refactored a little to avoid some duplication, but it
basically just adds `unrar-free` as a lower precedence alternative.

Tested with unrar-free 0.1.3.

Check the fields count before retrieving it not to go out of bounds.

This also slightly revise the logic to require the proper number of
fields in the RAR5 `attribute_field_with_space()` case.

Merge version matching for rar and unrar.  This is ever so slightly
slower in theory for unrar 5.x because there's an additional
unnecessary `sscanf()` call, but it's not actually gonna matter, and
saves quite a bit of duplicated logic.

This also makes the matching a bit more safe by verifying `sscanf()`
actually worked, and avoids using a uninitialized `version` value due
to an unexpected input.

Finally, this makes the parsing a bit more strict by requiring a
version number after the `RAR` and `UNRAR` line prefixes -- leading not
to using the uninitialized version variable.
This will be required by the upcoming unrar-free support as it reports
a `RAR archive ...` line that would have matched the `RAR` version
check.