Fix and upgrade the ahcpd package

Fix and upgrade the ahcpd package

- upgrades ahcpd to the latest release (0.50),
- adds librt dependency to the Makefile,
- removes the installation of the obsolete ahcp-dummy-config.sh file,
- fixes the ahcpd.init file to ensure compatibility with ahcpd 0.50,
  options, and provide a sane default behaviour (forward ahcpd messages
  without performing any configuration).

Signed-off-by: Gabriel Kerneis <kerneis@pps.jussieu.fr>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@18460 3c298f89-4303-0410-b956-a3cf2f4a3e73