Merge pull request #139 from abelbeck/fping6

Add backward compatibility for fping6 sym-link

Using sym-links to select IPv6 is discouraged (because it doesn't work the same for IPv4), but I understand that this can help for some use cases (i.e. in particular for the embedded linux distro use case, where having two binaries is an issue). Also, I guess nobody is going to have a binary called fping6 and expect it to do IPv4, so, I guess, no harm done.

diff --cc src/fping.c
Simple merge