Added -Wsentinel-in-midrule warning and 're2c:sentinel' configurations.

Added -Wsentinel-in-midrule warning and 're2c:sentinel' configurations.

The new analysis is meant to detect potential reads past the end of buffer
resulting from erroneous rules that allow sentinel symbol to occur in the
middle of lexeme. The analysis is only activated in cases bounds checking
is disabled with 're2c:yyfill:enable = 0;' and EOF rule is not used.

For example:

    ['] [^']* [']   { ... }

This may be a naive way to match single-quoted strings; it would crash the
lexer on input like "'aaa\0" (without the closing single quote).