Change type of MB_CUR_MAX and MB_CUR_MAX_L() to size_t.

POSIX requires that MB_CUR_MAX expands to an expression of type size_t.
It currently expands to an int. As these are already macros, don't
change the underlying type of these functions. There is no ned to touch
those.

Differential Revision: https://reviews.freebsd.org/D6645