gnutls-cli should handle network errors more gracefully

[Mantas Mikulėnas]

I often use `gnutls-cli` to connect to dualstack hosts (having both IPv4
and IPv6 addresses).

At times, when the client computer has an IPv6 default route, but the
server is temporarily unreachable over IPv6, `gnutls-cli` exits
immediately after failing to connect to the first address. Because of
this, it is impossible to connect to such servers until I disable IPv6
entirely on my computer. The lack of -4/-6 options to force IPv4 (or
IPv6) makes things only worse.

For example:

  $ gnutls-cli virgule.cluenet.org -p 636
  Processed 162 CA certificate(s).
  Resolving 'virgule.cluenet.org'...
  Connecting to '2a01:7e00::f03c:91ff:fe96:5efd:636'...
  Cannot connect to virgule.cluenet.org:636: No route to host
  $

The expected result here is that `gnutls-cli` would keep trying other
addresses returned from getaddrinfo(), until at least one succeeds.

-- 
Mantas Mikulėnas <grawity at gmail.com>