Merge new plugin check_ping_multiaddr.

This merges a new Nagios plugin, check_ping_multiaddr, which can
check reachability of multiple IP addresses, both IPv4 and IPv6
simultaneously, in a single call, using ICMP ECHO ("ping").

This merges the following commits:

 - f139bd20...  Add stub for new plugin check_ping_multiaddr.
 - 322268a9...  Implement address lookup in check_ping_multiaddr.
 - 20621ebd...  Perform actual pinging in check_ping_multiaddr.
 - 66cd3e87...  Configurable retries in check_ping_multiaddr.
 - 76970534...  Better status messages from check_ping_multiaddr.
 - a652e175...  Add a non-reflowing formatter for help texts.
 - 50027ec1...  Proper help text for check_ping_multiaddr plugin.
 - 68bb4638...  Update copyright year in relevant files.
 - f2fd2467...  Add fping to requirements of RPM and in README.