Mutually cross-reference sysexits(3) and err(3). Add text to err(3)

recommending that the standard exit codes in sysexits(3) be used.

Reviewed by:    ru

lib/libc/gen/err.3

@@ -158,6 +158,10 @@ and
 .Fn verrx
 functions do not return, but exit with the value of the argument
 .Fa eval .
+It is recommended that the standard values defined in
+.Xr sysexits 3
+be used for the value of
+.Fa eval .
 The
 .Fn err_set_exit
 function can be used to specify a function which is called before
@@ -209,7 +213,8 @@ if (error != 0)
 .Xr exit 3 ,
 .Xr fmtmsg 3 ,
 .Xr printf 3 ,
-.Xr strerror 3
+.Xr strerror 3 ,
+.Xr sysexits 3
 .Sh HISTORY
 The
 .Fn err

share/man/man3/sysexits.3

@@ -127,6 +127,7 @@ Something was found in an unconfigured or misconfigured state.
 The numerical values corresponding to the symbolical ones are given in
 parenthesis for easy reference.
 .Sh SEE ALSO
+.Xr err 3 ,
 .Xr exit 3 ,
 .Xr style 9
 .Sh HISTORY