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/print.c

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-3.0-only */
+
+#include <print.h>
+
+/* message print functions */
+static int __print_severity = MSCP_SEVERITY_WARN;
+
+void set_print_severity(int serverity)
+{
+	if (serverity < 0)
+		__print_severity = -1; /* no print */
+	__print_severity = serverity;
+}
+
+int get_print_severity()
+{
+	return __print_severity;
+}
+