Construct packager email from user- and hostname if not set through git config

The included RPM .spec file always needs a packager name or email address,
otherwise rpmbuild will complain. In properly configured git repos, the email
address configured with "git config user.email" would have been used so far,
but it should be possible to issue a "make rpms" without this requirement.

Thus we now fall back to constructing a most probably bogus email address by
combining the username and the hostname. This will allow to build RPMs for
personal purposes, however, for redistribution purposes one should always
explicitly configure a proper email address through "git config" beforehand.