modify configure.ac to build sheepdog on rhel5 environment

In our product machine we use rhel5 on 2.6.32-linux-kernel, but sheepdog can't build
on rhel5. It need some modification to do this.

1. old version autoconf does not support AM_SILENT_RULES() so replace it with check for shell.
2. use SIZEOF_SHORT/INT/LONG instead of AC_TYPE_INT16/32/64.
3. check header files such as sys/eventfd.h sys/signalfd.h etc.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Merge branch 'master' of github.com:sheepdog/sheepdog
dog: check the result of dog_exec_req() correctly

dog_exec_req doesn't return a result code after the commit 5940548.
We have to check rsp->result instead of its return value.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog: add more result check of dog_collie_exec()

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: don't remove vdi object from object list cache

We don't remove a vdi object from the underlying store even after the
vdi is deleted.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>