mbsstr: support GNULIB_MCEL_PREFER Support mcel API for apps that prefer it. * lib/mbsstr.c [GNULIB_MCEL_PREFER]: Include mcel.h not mbuiter.h. (mbchar_t, mb_equal) [GNULIB_MCEL_PREFER]: New type and function, to make it easier to use common code. (knuth_morris_pratt_multibyte): Don't assume mbchar_t's alignment is at least that of size_t. (knuth_morris_pratt_multibyte, mbsstr) [GNULIB_MCEL_PREFER]: Use mcel API. * modules/mbsstr (Depends-on): Add alignasof.