Currently, Fluentd core does not handle/refer Linux capability.
This class should handle/refer Linux capability.
Note that this class implementation is only working for Linux with
libcap-ng binding (capng_c) installed platform.
In other platform, this class only returns stubbed results.

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

in_tail should handle Linux capbilities.
In in_tail, File.readable? does not handles Linux capabilities.
So, we should handle them by libcap-ng binding wrapped class
which is Fluent::Capability.

On in_tail, the following capabilities should be handled:

* CAP_DAC_READ_SEARCH(:dac_read_search on capng_c)
* CAP_DAC_OVERRIDE(:dac_override on capng_c)

See also: https://man7.org/linux/man-pages/man7/capabilities.7.html

Thus, if ruby executable has CAP_DAC_OVERRIDE(:dac_override on
Fluent implementation) or CAP_DAC_READ_SEARCH (:dac_read_search on Fluentd
implementation) capability, Fluentd can read files even if they are
owned by root and 640 (-rw-r-----) mode.

e.g.) syslog log file:

```console
% ls -lh /var/log/syslog
-rw-r----- 1 syslog adm 49K 10月 30 16:26 /var/log/syslog
```

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>