Add exec_background option in plugin command info and a sudoers

option to match.  When set, commands are started in the background
and automatically foregrounded as needed.  There are issues with
some ill-mannered programs (like Linux su) so this is not the
default.

plugins/sudoers/def_data.c

@@ -350,6 +350,10 @@ struct sudo_defs_types sudo_defs_table[] = {
 	"limitprivs", T_STR,
 	N_("Set of limit privileges"),
 	NULL,
+    }, {
+	"exec_background", T_FLAG,
+	N_("Run commands on a pty in the background"),
+	NULL,
     }, {
 	NULL, 0, NULL
     }