Allow for standard depend syntax in the single parameter form of ruby_add_rdepend...

Allow for standard depend syntax in the single parameter form of ruby_add_rdepend and ruby_add_bdepend.

Make the two parameters form throw a warning for gentoo developers only (for now), and call back the
single-parameter function after wrapping. Update documentation to only suggest using the new syntax.

This allows dropping _ruby_add_rdepend and _ruby_add_bdepend convenience functions, and rather adds a
_ruby_wrap_conditions function.