Added support for personalization vector for dangling nodes

Hi,
Here is my attempt at adding the support for personalization vectors for dangling nodes. The default behaviour is implemented via R default function flags, although there is also some #ifdef code in prpack.cpp to provide a copy of the reset vector if the resetdangling vector is absent.

Testing is not yet thorough, but everything compiles and installs, checks pass and documentation updated.

Thanks for your help, and I hope this patch makes life easier for you.
Nope, let's not do it in R because igraph has higher level interfaces to other languages as well. It's not a problem if the default `u` vector is constructed in R, but let's not make the C core rely on the fact that the higher level interface will prepare the default `u` vector.