Warn if ttl is out of bounds
network: set ttl before opening server sockets

If TimeToLive is defined in collectd.conf after Server options, the
net_config_ttl value is updated after network_config_add_server and
the desired ttl is not used.
Merge pull request #654 from timl/fix_ttl_order

Fix ttl order
Fixing misplaced block in documentation.
Merge pull request #687 from txaj/fix-doc-postgresmodule-sslmode

Fixing misplaced block in documentation.