ahcpd 0.53

From [its homepage](http://www.pps.univ-paris-diderot.fr/~jch/software/ahcp/):

> AHCP is an autoconfiguration protocol for IPv6 and dual-stack IPv6/IPv4 networks designed to be used in place of router discovery and DHCP on networks where it is difficult or impossible to configure a server within every link-layer broadcast domain, for example mobile ad-hoc networks.

This PR is highly similar to my last one for `babeld`, #22892, because `ahcpd` and `babeld` are written by the same guy and often used together. `ahcpd` is available through `apt-get` on Debian and Ubuntu too.

There are no tests because `ahcpd` has no version/help flag, and needs to be run using `sudo`.