This adds some initial skeleton for local file subsystems of wClipboard.
The idea is to delegate handling of local file formats to dedicated
subsystems selected at runtime based on the compiled-in support code.
This is somewhat similar to the approach used by audin, rdpsnd, rdpgfx
channels in FreeRDP.
Only one subsystem is actually used by wClipboard during runtime. It is
selected by the ClipboardInitLocalFileSubsystem() function which will
try initializing the compiled-in subsystems in the preferred order. Thus
when adding new subsystems one must make sure to 1) return as soon as
any initialization succeeds, 2) leave wClipboard in usable state if the
initialization fails.
A POSIX file subsystem is added as a pioneer. It will handle local file
format "text/uri-list" and will use POSIX API to access the files. This
is the combination one would expect to be supported by Linux systems
which can run the XFreeRDP client.
The POSIX subsystem is enabled only when CMake detects <unistd.h> as
available. This is the core POSIX include file so we can reasonably
expect the rest of the POSIX API to be available along with that file.
We also define a new configuration option WITH_DEBUG_WCLIPBOARD which
will be used to guard some debug-only verbose logging in wClipboad.
