Added nxt_char_cast() macro for casting safely.

This macro is similar to C++'s static_cast().  It allows a very-limited
set of casts to be performed, but rejects most casts.  Basically, it
only allows converting from char to u_char and vice-versa, while
respecting the const qualifier.

/*
 * SYNOPSIS
 *      type char_cast(type, string);
 *
 * ARGUMENTS
 *      type    Type to which 'string' should be cast.
 *      string  String that should be cast to 'type'.  It should be a
 *              pointer, or a pointer to a pointer, to a character type,
 *              possibly const-qualified.
 *
 * DESCRIPTION
 *      This macro resembles C++'s static_cast().  It performs a cast,
 *      as if '(type) string' was written, but it performs additional
 *      checks that make sure the cast is limited to character types,
 *      that the input expression is also of a character type, and that
 *      it doesn't remove a const qualifier.
 *
 *      It also makes it easier to find all casts easily with grep(1).
 *
 * CAVEATS
 *      It doesn't allow using 'volatile'.  However, this isn't a big
 *      issue, since we don't use it with strings.  If necessary, the
 *      macro can be improved to support volatile.
 *
 * EXAMPLES
 *      char        *s = "foo";
 *      const char  *cs = "bar";
 *
 *      p = (char *) s;   /* OK.  Although unsafe, because it can be
 *                           confused with the case in the next line. */
 *      p = (char *) cs;  /* Very unsafe; it will probably cause
 *                           Undefined Behaviour. */
 *
 *      p = char_cast(char *, s);   // OK.
 *      p = char_cast(char *, cs);  // Compiler error.  Cast not allowed.
 *
 *      p = (const char *) s;   // OK.
 *      p = (const char *) cs;  // OK.
 *
 *      p = char_cast(const char *, s);   // OK.
 *      p = char_cast(const char *, cs);  // OK.
 *
 * SEE ALSO
 *      _Generic(3)
 */

Acked-by: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

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