fping 4.1

Bugfixes and other changes

• Fix problem when socket fd is 0 (#125, thanks Ramón Novoa!)
• Fix running on servers with disabled IPv6 (#118, thanks Simon Matter)
• Allow running "fping -h" or "--help" even when raw socket can't be opened (#131, thanks @teto)
• Fix build issue with FreeBSD and IPv6 (#132, thanks @gsnw)