Modeled after std::filesystem::operator/() from C++17.

To get udisks2 support started, this commit contains the configure.ac
option and a "neighbor" plugin which shows block devices.  Later, this
will allow mounting removable media with a new storage plugin.

From https://github.com/CM4all/libcommon/commit/a7cef404c1f7eedff1a629da48f77491218ee281

To be used by upcoming code.

These have no use, exception for creating an artificial dependency
from input_plugins[] to all archive plugins.

Prepare for option values.

Allow the caller to use a simple "for" loop without checking
arguments.

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

Closes #169

Use wchar_t for everything on Windows.  Solves a lot of filesystem
charset problems.

This commit adds the NeighborPlugin API which can be used to detect
nearby file servers that can be used by input plugins.  This list of
servers is exported using the new "listneighbors" command.  The idle
even "neighbor" notifies interested clients when a new neighbor is
found or an existing one is lost.

There's a lot missing currently: protocol&user documentation, and a
way to "mount" remote servers into the music database.  Obviously,
some code from the UPnP database plugin can be moved to a neighbor
plugin.