Replace lseek/fstat by ws_lseek64/ws_fstat64

lseek returns an off_t type which is system-dependent. Use ws_lseek64 in
favor of lseek as that supports 64-bit quanities.

Use ws_fstat64 instead of stat to support 64-bit file sizes on Windows.
For the majority of the changes, this makes no difference as they do not
apply to Windows ("ifndef _WIN32"; availability of st_blksize).

There are no other users of "struct stat" besides the portability code
in wsutil. Forbid the use of fstat and lseek in checkAPIs.

Change-Id: I17b930ab9543f21a9d3100f3795d250c9b9ae459
Reviewed-on: https://code.wireshark.org/review/3198
Reviewed-by: Guy Harris <guy@alum.mit.edu>