sheepfs: add check for libcurl in configure.ac

Check header and library for libcurl in configure.ac

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheepfs: add support for "rmdir" and "unlink"

Add support for "rmdir" and "unlink" so users can remove all directories
of accounts/containers and files of objects after using it.
This "remove" operation only remove local entry in sheepfs, the objects
are still in remote object-service.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
tests/084: add sheepfs http test

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>