shell: set output limit for single user instances

Problem: The KVS has a size limit of INT_MAX for when returning kvs
values.  This limit can be exceeded by a job's standard output because
it is continually appended and the total size is not yet tracked by
the KVS.  When reading the output later, such as via `flux job attach`,
this can lead to EOVERFLOW errors.

Solution: For a single user instance, default to a maximum standard
output of 1G instead of "unlimited".  1G should provide a practical
maximum for most users and encourage them to send standard output
to a file if they want to save excess standard output.  If desired,
the value can still be overwritten via the "output.limit" setting.

Fixes flux-framework#6256

src/shell/output.c

@@ -62,7 +62,8 @@
 #include "builtins.h"
 #include "log.h"
 
-#define MULTIUSER_OUTPUT_LIMIT "10M"
+#define SINGLEUSER_OUTPUT_LIMIT "1G"
+#define MULTIUSER_OUTPUT_LIMIT  "10M"
 
 enum {
     FLUX_OUTPUT_TYPE_TERM = 1,
@@ -1183,11 +1184,11 @@ static int get_output_limit (struct shell_output *out)
     json_t *val = NULL;
     uint64_t size;
 
-    /*  Set default to unlimited (0) for single-user instances,
+    /*  For single-user instances, cap at reasonable size limit.
      *  O/w use the default multiuser output limit:
      */
     if (out->shell->broker_owner == getuid())
-        out->kvs_limit_string = "0";
+        out->kvs_limit_string = SINGLEUSER_OUTPUT_LIMIT;
     else
         out->kvs_limit_string = MULTIUSER_OUTPUT_LIMIT;