dhcp_clientv4.ml would not build because of a type error;
copy_dhcp_chaddr needed to be used, instead of get_dhcp_chaddr, as an
argument to the function which generates a human-readable string from
the raw hex.

dhcpv4_option.ml now parses variable-length MTU options correctly,
although it seems that unikernels don't respect this value and will send
unfragmented packets of size greater than the transmitted MTU.

Some DHCP options require number parsing for buffers of specified sizes.
Of the DHCP options currently supported, only MTU parsing requires this;
however, future options may require it as well.