Change to use UVCHR_SKIP over UNI_SKIP

UNI_SKIP is somewhat ambiguous.  Perl has long used 'uvchr' as part of a
name to mean the unsigned values using the native character set plus
Unicode values for those above 255.

This also changes two calls (one in dquote_static.c and one in
dquote_inline.h) to use UVCHR_SKIP; they should not have been OFFUNI, as
they are dealing with native values.