string-util: simplify logic in strjoin_real()

string-util: simplify logic in strjoin_real()

The loops over (x, then all varargs, until a NULL is found) can be written much
simpler with an ordinary for loop. Just initialize the loop variable to x, test
that, and in the increment part, fetch the next va_arg(). That removes a level
of indentation, and avoids doing a separate strlen()/stpcpy() call for x.

While touching this code anyway, change (size_t)-1 to the more readable
SIZE_MAX.