Add --fwmark option for Linux

[deepkv]

This is based on tomangert's original outdated pull request.

--fwmark can be used in conjunction with ip rule and ip route to route ping packets based on the mark set by the aforementioned flag.

This has been supported by Linux since kernel version 2.6.25

Let me know if you'd like any changes.

[coveralls]

coverage: 81.69% (+0.1%) from 81.581%
when pulling c0cdb52 on deepkv:develop
into a3f4c57 on schweikert:develop.

[schweikert]

Thanks for your contribution!

Could you add a simple test to check at least that the program doesn't crash when --fwmark is passed? Pinging 12.0.0.7 is fine, and you don't need to check that the fwmark was actually set (that would be hard to do).

Also, please update doc/fping.pod (the man-page).

[deepkv]

Thanks for the review! Done.

[deepkv]

Apologies, it looks like setcap doesn't work in GitHub CI, which made the test fail. I've now used sudo instead. I believe it should work now.

[deepkv]

Update: the tests finally pass in GitHub CI in my repo after a couple more iterations. Thanks for your patience.