exterror: advertise error strings via kern.features.exterr_strings

Allow userland, in particular test cases for EXTERROR conversions,
to detect at run time whether extended errors include the
descriptive message strings, which depends on the EXTERR_STRINGS
kernel option and cannot be probed in any other way.

Reviewed by:		kib
MFC after:		1 week
Assisted-by:		Claude Code (Fable 5)
Differential Revision:	https://reviews.freebsd.org/D58321

share/man/man9/exterror.9

@@ -6,7 +6,7 @@
 .\" Konstantin Belousov <[email protected]> under sponsorship
 .\" from the FreeBSD Foundation.
 .\"
-.Dd November 5, 2025
+.Dd July 21, 2026
 .Dt EXTERROR 9
 .Os
 .Sh NAME
@@ -171,6 +171,17 @@ were modified to print the extended information if it is available
 in addition to the usual
 .Va errno
 decoding.
+.Pp
+Kernels built with the
+.Cd option EXTERR_STRINGS
+advertise the
+.Va kern.features.exterr_strings
+.Xr sysctl 8
+variable, see
+.Xr feature_present 3 ,
+indicating that the reported errors include the descriptive message strings.
+Without it, only the numeric information (error code, category,
+source line number and parameters) is available.
 .Sh ASYNCHRONOUS INPUT/OUTPUT
 Due to the nature of the
 .Fx

sys/kern/sys_generic.c

@@ -112,6 +112,10 @@ static MALLOC_DEFINE(M_IOCTLOPS, "ioctlops", "ioctl data buffer");
 static MALLOC_DEFINE(M_SELECT, "select", "select() buffer");
 MALLOC_DEFINE(M_IOV, "iov", "large iov's");
 
+#ifdef EXTERR_STRINGS
+FEATURE(exterr_strings, "Extended error reporting includes message strings");
+#endif
+
 static int	pollout(struct thread *, struct pollfd *, struct pollfd *,
 		    u_int);
 static int	pollscan(struct thread *, struct pollfd *, u_int);