gateway=server:255.255.255.0 because that is the perferred format,
and the sys/libsa code already knows how to parse that format.
(Copied ip_convert here from the libsa code.)
Sun RPC bootparam/whoami return by requesting a "pseudo file"
named "gateway" and using its contents as the gateway:netmask
Example /etc/bootparams line: client gateway=router:0xfffffff0