Fix MinGW build warnings.

Conditionally define ENOENT, EINVAL, etc. (was unconditional).

Add/use PRIzu, PRIzd, and PRIzx for use in malloc_printf() calls.  gcc issued
(harmless) warnings since e.g. "%zu" should be "%Iu" on Windows, and the
alternative to this workaround would have been to disable the function
attributes which cause gcc to look for type mismatches in formatted printing
function calls.