signed int.
(sftp_callback_func): New typedef.
(struct sftp_callback): Changed prototype for nextfun. Changed
UINT64 to off_t.
Changed all prototypes to not pass and return structs without
using pointers.

* src/sftp/sftp_c.c: General change of calling conventions. Pass
struct * and const struct *, instead of passing and returning
structures "by-value".

Rev: src/sftp/sftp_c.c:1.2
Rev: src/sftp/sftp_c.h:1.2