There's no point in passing a LightSong reference here; the callee is
interested only in the URI.

All remaining events are specific to the Partition.

Use MaskMonitor to eliminate duplicate code.

Avoid wasting 4 kB stack per directory level.

There were two ways this could fail:

1. division by zero when sysconf(_SC_PAGESIZE)==0

2. mmap() failure because the size parameter is not aligned to page
   size

Neither ever happened: sysconf() never fails, and the only caller
passes a size that is already aligned.  Phew.

The old check

 unsigned(value) > std::numeric_limits<unsigned>::max()

.. cannot ever fail.

Move the GlobalEvents::Monitor instance into class Instance.
Eliminate all global variables.

Use walk!=nullptr instead.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().