src/*.h: Improve.

* src/auth.h, src/channel-func.h, src/channel-type.h, src/error.h,
  src/key-func.h, src/key-type.h, src/session-func.h,
  src/session-type.h: Mark prototypes as external functions.
  Simplify SCM functions arg list.
src/key-func.c: Add comments.
src/auth.c: Fix comment.
src/channel-func.c: Add checks for smob type.

* src/channel-func.c (guile_ssh_channel_is_eof_p)
  (guile_ssh_channel_is_open_p, guile_ssh_channel_close)
  (guile_ssh_channel_read, guile_ssh_channel_pool)
  (guile_ssh_channel_request_env, guile_ssh_channel_request_exec)
  (guile_ssh_channel_open_session): Add check for smob type.