regcomp.c: Introduce vFAIL2utf8f to replace the APPLY(X,Y) hack

The APPLY() hack was added in the previous commit to get
vFAIL4("%"UTF8f, UTF8fARG(a,b,c)) working.  Without it,
vFAIL4 would complain about missing arguments, since it only
saw one, but if replaced with vFAIL2, then Simple_vFAIL2 would
complain about having too many arguments.

This commit introduces a vFAIL2utf8f macro that works around
this and enables us to remove the hack.
t/re/reg_mesg.t: Tests for latin1 error messages/warnings
regcomp.c, S_nextchar: Use UTF8SKIP if UTF is true