character.h: Rename locals to avoid shadowing.

character.h: Rename locals to avoid shadowing.

* character.h (PREV_CHAR_BOUNDARY, FETCH_STRING_CHAR_ADVANCE):
(FETCH_STRING_CHAR_AS_MULTIBYTE_ADVANCE, FETCH_CHAR_ADVANCE):
(FETCH_CHAR_ADVANCE_NO_CHECK, INC_POS, DEC_POS, BUF_INC_POS):
(BUF_DEC_POS): Be more systematic about renaming local temporaries
to avoid shadowing.