Bug 4796: comm.cc !isOpen(conn->fd) assertion when rotating logs (#382)

Bug 4796: comm.cc !isOpen(conn->fd) assertion when rotating logs (#382)

Squid abandoned cache.log file descriptor maintenance, calling fd_open()
but then closing the descriptor without fd_close(). If the original file
descriptor value was reused for another purpose, Squid would either hit
the reported assertion or log a "Closing open FD" WARNING (depending on
the new purpose). The cache.log file descriptor is closed on log
rotation and reconfiguration events.

This short-term solution avoids assertions and WARNINGs but sacrifices
cache.log listing in fd_table and, hence, mgr:filedescriptors reports.

The correct long-term solution is to properly maintain descriptor meta
information across cache.log closures/openings, but doing so from inside
of debug.cc is technically difficult due to linking boundaries/problems.