Allow cmdgen to read keys from standard input.

This reworks the cmdgen main program so that it loads the input file
into a LoadedFile right at the start, and then every time it needs to
do something with the contents, it calls one of the API functions
taking a BinarySource instead of one taking a Filename.

The usefulness of this is that now we can read from things that aren't
regular files, and can't be rewound or reopened. In particular, the
filename "-" is now taken (per the usual convention) to mean standard
input.

So now you can pipe a public or private key file into cmdgen's
standard input and have it do something useful. For example, I was
recently experimenting with the SFTP-only SSH server that comes with
'proftpd', which keeps its authorized_keys file in RFC 4716 format
instead of the OpenSSH one-liner format, and I found I wanted to do

  grep 'my-key-comment' ~/.ssh/authorized_keys | puttygen -p -

to quickly get hold of my existing public key to put in that file. But
I had to go via a temporary file to make that work, because puttygen
couldn't read from standard input. Next time, it will be able to!

ssh.h

@@ -1260,6 +1260,7 @@ void ssh2_write_pubkey(FILE *fp, const char *comment,
 char *ssh2_fingerprint_blob(ptrlen);
 char *ssh2_fingerprint(ssh_key *key);
 int key_type(const Filename *filename);
+int key_type_s(BinarySource *src);
 const char *key_type_to_str(int type);
 
 bool import_possible(int type);