Convert usages of errx to h_platform_errx

This BSD call will have to be implemented on windows. Right now
we have a stub that just exits the process.

src/platform_bsdlike.c

@@ -0,0 +1,10 @@
+#include "platform.h"
+
+#include <err.h>
+#include <stdarg.h>
+
+void h_platform_errx(int err, const char* format, ...) {
+  va_list ap;
+  va_start(ap, format);
+  verrx(err, format, ap);
+}