* dh_shlibdeps: Update documentation for -L and -l. dpkg-shlibdeps is now

* dh_shlibdeps: Update documentation for -L and -l. dpkg-shlibdeps is now
  much smarter, and these options are almost never needed. Closes: #459226
* dh_shlibdeps: If a relative path is specified in -l, don't prepend the pwd
  to it, instead just prepend a slash to make it absolute. dpkg-shlibdeps
  has changed how it used LD_LIBRARY_PATH, so making it point into the
  package build directory won't work.