Prefer idx_t to size_t in system.c

Prefer idx_t to size_t in system.c

* src/buffer.c (_flush_write): Return idx_t, not ssize_t,
to accommodate system.c changes.  All uses changed.
(_gnu_flush_write): Output correct errno value after write error.
Simplify multi-volume mode.
* src/system.c (sys_write_archive_buffer)
(sys_child_open_for_compress, sys_exec_setmtime_script):
Prefer idx_t to size_t.