Change dhcp_request() and v4_config() to be able to configure dhcp

Change dhcp_request() and v4_config() to be able to configure dhcp
for an interface without an active network connection.

- Don't write options to dhclient.conf that are default. Only use
  the 'host-name' option for hostname associated dhcp requests.

- Run dhclient with options to reduce the time it takes to finish
  in case it gets no answer from a DHCP server.

- Don't bother removing the interface from group dhcp and save the
  configuration files even if dhclient fails. If the same interface
  is statically configured later, it is removed from the group dhcp.

- Add 'dhcp' to hostname.if file regardless whether dhcp_request()
  was successful or not.

- Change the Netmask question to be more autoinstaller friendly.

based on krw's work
discussed with and OK krw@