Large file support, mkstemp, and off_t

[Allen Webb]

We are trying to enable large file support for everything built for
ARM on ChromeOS and openssh uses mkstemp which has a 64-bit version
for compatibility with open64. When looking into enabling the large
file support flags, I noticed:

`void    start_progress_meter(const char *, off_t, off_t *);`

in progressmeter.h which would have different ABIs with and without
large file support. You all probably ought to avoid exposing off_t in
any external-facing APIs.

Allen