genconfig: Add client-config-dir

* Add client config directory for client specific configuration options
  such as IP addresses.
genconfig: Expand the subnet

* Use a larger subnet (2x the size) to allow for more hard-coded
  configurations.
genconfig: Use servername if $1 not specified

* Set the common name to servername set during last ovpn_init if $1 is
  not passed in.
* Simplies re-running ovpn_genconfig when features are added.
genconfig: Backup old config file

* Backup previous config file before overwriting.
run: Always ensure client dir exists

* OpenVPN will fail to start if this directory doesn't exist.