common: Fix flushing copy buffers.
* common/exectool.c (copy_buffer_flush): Write and flush the data, but do not hide EAGAIN from the caller. (gnupg_exec_tool_stream): Retry on EAGAIN.
- GnuPG-bug-id: T2425
- Signed-off-by: Justus Winter <justus@g10code.com>