Pageant: factor out cross-platform parts of add_keyfile().

Pageant: factor out cross-platform parts of add_keyfile().

I've now centralised into pageant.c all the logic about trying to load
keys of any type, with no passphrase or with the passphrases used in
previous key-loading actions or with a new user-supplied passphrase,
whether we're the main Pageant process ourself or are talking to
another one as a client. The only part of that code remaining in
winpgnt.c is the user interaction via dialog boxes, which of course is
the part that will need to be done differently on other platforms.