update-alternatives - maintain symbolic links determining default commands

By default alternatives uses auto mode picking the “best” alternative.

TODO: how to enforce sysadmin preference

from phar-php example:

Requires:      alternatives
...
 
%install
...
# touch for ghost
%{__rm} $RPM_BUILD_ROOT%{_bindir}/phar
touch $RPM_BUILD_ROOT%{_bindir}/phar
 
%post phar
update-alternatives \
       --install %{_bindir}/phar phar %{_bindir}/phar%{ver_suffix}.phar %{ver_suffix} \
       --slave %{_mandir}/man1/phar.1 phar.1 %{_mandir}/man1/phar%{ver_suffix}.1* || :
 
%postun phar
if [ $1 -eq 0 ]; then
       update-alternatives --remove phar %{_bindir}/phar || :
fi
 
...
%files
...
%ghost %{_bindir}/phar
%ghost %{_mandir}/man1/phar.1

1. add Requires: alternatives to runtime deps
2. the path must be ghost and regular file, otherwise rpm will complin about file-conflicts
3. use --install to setup the symlinks in package %post scriptlet.
4. the --slave option is optional and may be repeated, use if you wish to “group” alternatives
5. use proper %postun to remove alternative on package uninstall