fileio: set O_NOCTTY when reading virtual files

Better be safe than sorry, maybe someone points this call to a TTY one
day, and we'd rather not make it our controlling TTY in that case.

src/basic/fileio.c

@@ -395,7 +395,7 @@ int read_virtual_file(const char *filename, size_t max_size, char **ret_contents
          * contents* may be returned. (Though the read is still done using one syscall.) Returns 0 on
          * partial success, 1 if untruncated contents were read. */
 
-        fd = open(filename, O_RDONLY|O_CLOEXEC);
+        fd = open(filename, O_RDONLY|O_NOCTTY|O_CLOEXEC);
         if (fd < 0)
                 return -errno;