Added global default DHCP settings and modified documentation to reflect.

As discussed in Arch bug report 35668 (https://bugs.archlinux.org/task/35668?project=1&cat%5B0%5D=31&string=netctl), I added a DHCP options conf file to /etc/netctl/hooks/dhcp.conf and modified the manpages to reflect these changes. The dhcp.conf file is also documented.

I see no need to symlink the dhcp.conf file to /etc/ or /etc/netctl. The documentation provides  sufficient information regarding what settings can be modified and how.

I don't know how this squares with Arch's preferred location of package .conf files, but I agree with Jouke that doing it this way is the simplest, as it does not require any modifications to the code (unlike my previous approach).