Network: add helpers to resolve hostnames to IPs

Network: add helpers to resolve hostnames to IPs

Add addr_to_ip and get_ip_from_hostname helpers to PVE::Network

The first helper, addr_to_ip, is based on Wolfgangs version of this
[0]
I just moved it from PVE::Tools to PVE::Network, as it seems a more
fitting place.
It uses getnameinfo to extract information from the paddr parameter,
which is sockaddr struct
It gets used in the second helper and in a bug fix series from
Wolfgang [1]

The second helper, get_ip_from_hostname, resolves an hostname to an
IP and checks if it isn't one from the for loopback reserved 127/8
subnet. It will be used in get_remote_nodeip from PVE::CLuster and
for a bugfix in pvecm.

[0]: http://pve.proxmox.com/pipermail/pve-devel/2017-April/026099.html
[1]: http://pve.proxmox.com/pipermail/pve-devel/2017-April/026098.html

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>