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/libstore/unix/build/local-derivation-goal.cc

@@ -1993,7 +1993,7 @@ void LocalDerivationGoal::runChild()
             throw SysError("changing into '%1%'", tmpDir);
 
         /* Close all other file descriptors. */
-        unix::closeMostFDs({STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO});
+        unix::closeExtraFDs();
 
 #if __linux__
         linux::setPersonality(drv->platform);