add -Wshadow to CXXFLAGS too

This one can be a bit annoying at time, but since I fixed few problems
caused by this recently, let's try going with it.
require capng on linux by default

It's safer to have the daemon not keeping the root privileges,
so do not silently ignore it if capng is not available, those
who do not want it for some reason have to disable it explicitly.
remove useless rsync code

Apparently it doesn't do anything, was introduced
in 1bd7d4a9f6cd56eb239ea2a5615a6e0cf798a96c but wasn't made to do anything
useful.
remove use of glibc backtrace()

It's not used anywhere, it might be added manually while developing,
but on today's systems it actually doesn't print out anythin guseful anyway.