sysconf(3): add _SC_UEXTERR_LEN, the max length of the extended error string

Reviewed by:	brooks
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D50483

include/unistd.h

@@ -292,6 +292,7 @@ typedef	__useconds_t	useconds_t;
 #define	_SC_NPROCESSORS_CONF	57
 #define	_SC_NPROCESSORS_ONLN	58
 #define	_SC_CPUSET_SIZE		122
+#define	_SC_UEXTERR_MAXLEN	123 /* user */
 #endif
 
 /* Extensions found in Solaris and Linux. */

lib/libc/gen/sysconf.c

@@ -34,6 +34,7 @@
 
 #include "namespace.h"
 #include <sys/param.h>
+#include <sys/exterrvar.h>
 #include <sys/time.h>
 #include <sys/sysctl.h>
 #include <sys/resource.h>
@@ -602,6 +603,8 @@ sysconf(int name)
 			return (-1);
 		return ((long)value);
 #endif
+	case _SC_UEXTERR_MAXLEN:
+		return (UEXTERROR_MAXLEN);
 
 	default:
 		errno = EINVAL;

sys/sys/exterrvar.h

@@ -26,6 +26,8 @@ struct uexterror {
 	char msg[128];
 };
 
+#define	UEXTERROR_MAXLEN	256
+
 #define	UEXTERROR_VER		0x10010001
 
 #define	EXTERRCTL_ENABLE	1