libutil: rename and optimize closeMostFDs

this is only used to close non-stdio files in derivation sandboxes. we
may as well encode that in its name, drop the unnecessary integer set,
and use close_range to deal with the actual closing of files. not only
is this clearer, it also makes sandbox setup on linux fast by 1ms each

(cherry-picked and adapted from
c7d97802e4)

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>
Co-authored-by: John Ericson <git@JohnEricson.me>

src/libutil/file-descriptor.hh

@@ -143,10 +143,10 @@ public:
 namespace unix {
 
 /**
- * Close all file descriptors except those listed in the given set.
+ * Close all file descriptors except stdio fds (ie 0, 1, 2).
  * Good practice in child processes.
  */
-void closeMostFDs(const std::set<Descriptor> & exceptions);
+void closeExtraFDs();
 
 /**
  * Set the close-on-exec flag for the given file descriptor.