use gnulib to print --help and --version

* Makefile.am: add empty MAINTAINERCLEANFILES variable.
* bootstrap.conf: include dirname, propername, and version-etc-fsf modules
* configure.ac: remove COPYRIGHT_YEAR from config.h
* po/POTFILES.in: add lib/version-etc.c to translations
* src/hello.c: remove print_version(), and rename print_help() to usage()
* src/system.h: add inline functions to print version and usage info
use standard error printing facilities

* bootstrap.conf: add error module
* src/hello.c: use error() where needed