generator only if needed (i.e. if the key is going to be
encrypted), and let the --server determine which seed-file is
used.

* src/lsh-writekey.c (main): New option --server, that writes the
output files where the server expects its host key.

* src/lsh-writekey.c (main): Check that files doesn't exist before
reading the key, but don't create the files until they are
actually written to.
(file_exists): New fucntion.
(check_file): New function.
(open_file): New function.
(open_private_file): Deleted function.
(open_public_file): Deleted function.

Rev: src/lsh-writekey.c:1.25