Add support for IPv6 addresses to the pNFS "-p" option.

This patch adds code to handle IPv6 addresses returned by getaddrinfo()
for the host entries in the "-p" command line argument.
If the IPv6 address is a link local address, only use it if it is the
only address for the host. This is done since there is no way to know
if the NFSv4.1 pNFS client is in the same scope zone as the MDS.
inet_ntop() is used for the IPv6 address translation, since the client
will have no use for the scope zone suffix and inet_ntop() does not
put this in the address string.

Discussed with: bu7cher@yandex.ru