Eliminate the RPM_I18NSTRING_TYPE data type

Remove all support for RPM_I18NSTRING_TYPE in RPM (and there
may be some associative array like methods in bindings). The side
effect there will be to change the data type in packaging, which
has no legacy compatibility issues with new packages read by old rpm.

The reverse "backward" compatibility can be dealt with by changing
the data type on the fly (which is tricky because header blobs
are digitally signed, but the place where the type conversion needs
to be accomplished is well known from a similar
 RPM_CHAR_TYPE -> RPM_UINT8_TYPE