make (system-type 'machine) not depend on the secutiry guard

Allow `system-type` on non-Windows platforms to run `uname` to get
machine information, even in a sandbox or other contexts with a
limiting secutiry guard.

pkgs/racket-test-core/tests/racket/file.rktl

@@ -1648,6 +1648,11 @@
     (err/rt-test (udp-connect! early-udp "localhost" 40000)  (net-reject? 'udp-connect! "localhost" 40000 'client))
     (err/rt-test (udp-send-to early-udp "localhost" 40000 #"hi")  (net-reject? 'udp-send-to "localhost" 40000 'client))))
 
+;; Interaction with `system-type` - - - - - - - - - - - - - - - - - - -
+
+(parameterize ([current-security-guard (make-file-sg '())])
+  (test #f regexp-match? "unknown machine" (system-type 'machine)))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Check `in-directory'
 

racket/src/racket/src/string.c

@@ -6063,6 +6063,19 @@ void machine_details(char *buff)
 {
   Scheme_Object *subprocess_proc;
   int i;
+  Scheme_Config *config;
+  Scheme_Security_Guard *sg;
+  Scheme_Cont_Frame_Data cframe;
+ 
+  /* Use the root security guard so we can test for and run
+     executables. */
+  config = scheme_current_config();
+  sg = (Scheme_Security_Guard *)scheme_get_param(config, MZCONFIG_SECURITY_GUARD);
+  while (sg->parent) { sg = sg->parent; }
+  config = scheme_extend_config(config, MZCONFIG_SECURITY_GUARD, (Scheme_Object *)sg);
+
+  scheme_push_continuation_frame(&cframe);
+  scheme_install_config(config);
 
   subprocess_proc = scheme_builtin_value("subprocess");
 
@@ -6092,12 +6105,16 @@ void machine_details(char *buff)
 	  buff[--c] = 0;
 	}
 
+        scheme_pop_continuation_frame(&cframe);
+
 	return;
       }
     }
   }
 
   strcpy(buff, "<unknown machine>");
+
+  scheme_pop_continuation_frame(&cframe);
 }
 #endif