Do not use private macros for special types

Do not use private macros for special types

When you need to write a callback for archive_read_open2, you must provide a
function which has the same signature as the typedef. The documentation says
that the read callback returns a ssize_t, but on windows there is no such
ssize_t. libarchive falls back on another type and use the private macro
__LA_SSIZE_T, thus it is not possible to write portable code that use
read_open2.

This patch replaces that macro with a simple typedef la_ssize_t (and does the
same for la_int64_t).