dpkg-shlibdeps: new warning about useless libraries, new --warnings option

dpkg-shlibdeps: new warning about useless libraries, new --warnings option

* scripts/dpkg-shlibdeps.pl: Let the user configure the set of
warnings that he wants. Add a new warning for useless libraries
on all binaries analyzed. Disable by default the warning that
does the same but individually, for each binary analyzed.
* man/dpkg-shlibdeps.1: Update manual page accordingly.