fd leak detector

[Dean Gaudet <dg...@arctic.org>]

In case someone has a few spare cycles... it'd be cool if the child accept
loop noted what fd it gets from accept() and complained if the fd changed
between iterations.  This is potentially caused by an fd leak -- but isn't
always -- if some modules, for example, delay opening connections to
databases/etc. they may end up allocating a long lived fd during a
connection.

Dean