Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

These events are relevant for all partitions.

Eliminate global variables.

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

Lists files and directories.  Supports storage plugins.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Add the new idle event to Idle.hxx/Idle.cxx.