Move half of Pageant out into a cross-platform source file.

Move half of Pageant out into a cross-platform source file.

I'm aiming for windows/winpgnt.c to only contain the parts of Windows
Pageant that are actually to do with handling the Windows API, and for
all the actual agent logic to be cross-platform.

This commit is a start: I've moved every function and internal
variable that was easy to move. But it doesn't get all the way there -
there's still a lot of logic in add_keyfile() and get_keylist*() that
would be good to move out to cross-platform code, but it's harder
because that code is currently quite intertwined with details of
Windows OS interfacing such as printing message boxes and passphrase
prompts and calling back out to agent_query if the Pageant doing that
job isn't the primary one.