Convert Perl_sv_pos_u2b_proper() to Perl_sv_pos_u2b_flags().

Convert Perl_sv_pos_u2b_proper() to Perl_sv_pos_u2b_flags().

Change from a value/return offset pointer to passing a Unicode offset, and
returning a byte offset. The optional length value/return pointer remains.
Add a flags argument, passed to SvPV_flags(). This allows the caller to
specify whether mg_get() should be called on sv.