refactor error message-related functions

split message print fuctions (mpr_*), strerrno, and mscp_get/set_error
into print.c/h and strerrno.c/h.

ToDo: revise usages of priv_set_errv and pr_* functions.

src/minmax.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-3.0-only */
+#ifndef _MINMAX_H_
+#define _MINMAX_H_
+
+#define min(a, b) (((a) > (b)) ? (b) : (a))
+#define max(a, b) (((a) > (b)) ? (a) : (b))
+
+#endif /* _MINMAX_H_ */