regcomp.c: Refactor join_exact() to handle all multi-char folds

regcomp.c: Refactor join_exact() to handle all multi-char folds

join_exact() prior to this commit returned a delta for 3 problematic
sequences showing that the minimum length they match is less than their
nominal length.  It turns out that this is needed for all
multi-character fold sequences; our test suite just did not have the
tests in it to show that.  Tests that do show this will be added in a
future commit, but code elsewhere must be fixed before they pass.
regcomp.c