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().

Apparently all other C libraries are not compatible with "constexpr".
Those which are not will get a performance penalty, but at least they
work at all.

This is necessary to avoid an assertion failure in UpdateQueuedSong()
after LockSeek() has failed.

Don't need to clear the caller-specified one.

playlist_domain is no longer used for class Error.