shell: inherit FLUX_F58_FORCE_ASCII from job

Problem: If a user sets FLUX_F58_FORCE_ASCII for a job, then job
artifacts created by the job shell, such as environment variables,
directories, and output files, should obey this variable and use ascii
encoded F58.  However, this is currently not the case since the job
shell is using the environment inherited from the broker and/or IMP,
not the job environment.

Set up the job shell to inherit a subset of environment variables
from the job environment early during startup. At first, add just
FLUX_F58_FORCE_ASCII to this list.

Author: grondo

src/shell/shell.c

@@ -1288,6 +1288,25 @@ static int shell_register_event_context (flux_shell_t *shell)
     return 0;
 }
 
+/*  Export a static list of environment variables from the job environment
+ *   to the current shell environment. This is important for variables like
+ *   FLUX_F58_FORCE_ASCII which should influence some shell behavior.
+ */
+static int shell_export_environment_from_job (flux_shell_t *shell)
+{
+    const char *vars[] = {
+        "FLUX_F58_FORCE_ASCII",
+        NULL,
+    };
+
+    for (int i = 0; vars[i] != NULL; i++) {
+        const char *val = flux_shell_getenv (shell, vars[i]);
+        if (val && setenv (vars[i], val, 1) < 0)
+            return shell_log_errno ("setenv (%s)", vars[i]);
+    }
+    return 0;
+}
+
 int main (int argc, char *argv[])
 {
     flux_shell_t shell;
@@ -1325,6 +1344,9 @@ int main (int argc, char *argv[])
     if (!(shell.info = shell_info_create (&shell)))
         exit (1);
 
+    if (shell_export_environment_from_job (&shell) < 0)
+        exit (1);
+
     if (shell_register_event_context (&shell) < 0)
         shell_die (1, "failed to add standard shell event context");