New browser-plugins

Technical implementation
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browser-plugins(%{_target_base_arch}) provides:
- %dir /etc/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.

update-browser-plugins script knows about browsers. it has hardcoded list of
browser plugin directories.

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.
- %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