Use installed DHCP client as default

Currently, netctl uses dhclient only if it's explicitly specified in $DHCPClient. Since both supported DHCP clients are listed equally as optional dependencies, I would have expected that dhclient is automatically used if dhcpcd is not installed.

I attached a patch which checks for installed DHCP clients and uses the first one it finds as default.

I'm not an expert bash coder and only had a quick look at the netctl source; I probably didn't grasp all coding conventions and the like. I'll gladly improve my patch according to your suggestions :-)