Support for IPv6 has been broken with revision 1522: Because IPv6-addresses

Support for IPv6 has been broken with revision 1522: Because IPv6-addresses
contain colons, simply checking for a colon and using everything after it does
destroy correctly formatted IPv6-addresses.

This patch checks for dots '.' in the address. If the address contains at least
one dot, it is considered to be a hostname or an IPv4-address and a simple
search for a colon is done.

If no dot is found, the code will check for an opening square bracket '[' at
the beginning of the address. If one if found, the format
  [address]:port
is assumed.

If neither applies, the default port will be used.
-- Florian Forster

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@1523 a5681a0c-68f1-0310-ab6d-d61299d08faa