cli: Split out platform-specific tty code for Windows/Linux

Use common entry point names for the Linux and Win32 implementations;
add Win32 stubs.  Avoid including <termios.h> on Windows, as it breaks
the build.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
cli: Move fork_and_wait() into platform-specific sections

Windows doesn't have a <sys/wait.h>, and subprocess handling is likely
to differ quite a bit from Linux.  So let's move this out of the way
for now, so we can build the CLI for Linux (sans QR encoding support).

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
gui: Look for *.ui files in current dir on Windows

We do not yet have a good place to install these files, so just look
for them in the working directory.  This allows stoken-gui to start
up and display tokencodes.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>