Do log output to stderr while running dhcpd(8) in foreground to make

Do log output to stderr while running dhcpd(8) in foreground to make
behaviour in accordance with man page. Introduce '-v' option to make
output more verbose.

Do a little refactoring to make code more consistent with other daemons
like ospfd(8), httpd(8), relayd(8), etc.

Feedback from bluhm benno

ok bluhm