Guard exit routine against PID.

The new-style atexit implementation used /also/ propagates across fork
calls, so every child which is forked ends up calling finish(). This is
marginally annoying except for Subversion modules, where the
'module_has_conflict' check run by log_command() technically involves a
forked child for the implementation.

Since the abnormal-end finish that gets run by atexit takes priority
over the result code from module_has_conflict it appears that there's
always a conflict.

Instead, just make sure that finish() is only called for real from the
process that installed the handler in the first place.

kdesrc-build

@@ -7562,7 +7562,8 @@ eval
     }
     else
     {
-        push @atexit_subs, sub { finish($ctx, 99) };
+        my $curPid = $$;
+        push @atexit_subs, sub { finish($ctx, 99) if $$ == $curPid };
     }
     # }}}