Fix for macOS arp

[raymanP]

Tested and working on macOS.

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Summary of changes:

• selects arp regexp by global DARWIN variable instead of testing variants (_regex_std, _regex_darwin), which was not fail proof

[GhostofGoes]

Thanks for the fix, and apologies for the delay in approving it. I'll try to get a release out tomorrow while sitting around in the airport.

[raymanP]

Thank you for merging!
Some tests seem to fail under Linux, e.g. in test_methods.py the test_arp_various_args.
There it seem to test under Linux some macOS things. Don't know if it was running before right.

FAILED tests/test_methods.py::test_arp_various_args[58:6d:8f:07:c9:94-192.168.1.1-OSX/arp_-an.out] - AssertionError: assert None == '58:6d:8f:7:c9:94'
FAILED tests/test_methods.py::test_arp_various_args[58:6d:8f:07:c9:94-192.168.1.1-OSX/arp_-a.out] - AssertionError: assert None == '58:6d:8f:7:c9:94'
FAILED tests/test_methods.py::test_arp_various_args[52:54:00:12:35:02-10.0.2.2-solaris10/arp_10-0-2-2.out] - AssertionError: assert None == '52:54:0:12:35:2'

[raymanP]

I added a PR for fixing the test. Now should be good :)