New browser-plugins

Technical implementation
~~~~~~~~~~~~~~~~~~~~~~~~

browser-plugins(%{_target_base_arch}) provides:
- %dir /etc/browser-plugins
- %dir %{_plugindir}
%{_plugindir} expands to %{_libdir}/browser-plugins

- %{_sbindir}/update-browser-plugins script
The script will parse /etc/browser-plugins/*.blacklist and delete dead and add
new symlinks to browser specific plugin directories.

The list of browser plugin directories is taken by reading existing symlinks
matching glob: /etc/browser-plugins/browser.d/<browser>.%{_target_base_arch}

plugin provides
- %config(noreplace) /etc/browser-plugins/<plugin>-<browser>-%{_target_base_arch}.blacklist
The blacklist contains shell globs of filenames to be excluded from linking to
browser specific plugin directory. The shell globs are base of %{_plugindir},
subdirectories are allowed. Comments and empty lines are ignored.

- %post/%postun scriptlet
the scriptlet will call %update_browser_plugins

browser provides
- /etc/browser-plugins/browser.d/<browser>.%{_target_base_arch} symlink
the symlink will point to the single directory where it searches for its plugins.

- %post/%postun scriptlet
the scriptlet will call %update_browser_plugins