Make the name of env variables variable

[driesmp]

Right now, "new_dhcp6_ia_pd1_prefix1" and "old_dhcp6_ia_pd1_prefix1" is hardcoded, it should be overwritable by specifying it as a command line argument, like "-old-prefixname" and "-new-prefixname" to make it more suitable for others to use it. The old and new prefix name has a default value equal to the current hardcoded one. getopt() seems the best here.