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.h

@@ -0,0 +1,18 @@
+#ifndef HAMMER_PLATFORM__H
+#define HAMMER_PLATFORM__H
+
+/**
+ * @file interface between hammer and the operating system /
+ * underlying platform.
+ */
+
+#include "compiler_specifics.h"
+
+/* Error Reporting */
+
+/* BSD errx function, seen in err.h */
+H_MSVC_DECLSPEC(noreturn) \
+void h_platform_errx(int err, const char* format, ...)	\
+  H_GCC_ATTRIBUTE((noreturn, format (printf,2,3)));
+
+#endif