dhcp: adjust initscripts and misc details

Revamp initscripts see bug #3601
* Trim whitespace
* Check for installed binary (dhcp-relay)
* Check for installed config (dhcp-server)
* Check for proper variable setup from the script (both)
* Add "FAIL" message when startup fails (blind OK isn't accurate)
* Add start/stop messages to dhcp-relay

Switch bool to "dhcp (ISC)" to keep in line with package sorting.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>