toke.c: "Unrecognized character" error shouldn't leak memory or mangle characters.

This error could've leaked memory if used within as eval, and could
cut off in the middle of a character, leaving worthless binary data
in the error message.
regcomp.c, S_compile_runtime_code: croak using the SVf format

This makes error messages from code like this:

    $f = "(?{q\0foo\0 + \x{FFFF}})";
    "a" =~ /^a$f/;

Be both UTF-8 and nul clean.
regcomp.c: Make all warnings and error messages UTF-8 clean
regcomp.c, S_nextchar: Use UTF8SKIP if UTF is true