Fix support for FUSE_POSIX_ACL

Fix support for FUSE_POSIX_ACL

The kernel may set the FUSE_POSIX_ACL flag in the FUSE_INIT request to
notify the userspace daemon that the OS does support POSIX ACLs for FUSE
file systems. If the filesystem implementation wants to enable POSIX
ACLs, it has to reply with the FUSE_POSIX_ACL flag set. However, the
reply to the kernel never includes this flag, even if the implementation
expresses the need by setting the FUSE_CAP_POSIX_ACL flag in the
fuse_conn_info::want variable passed to its init callback. We modify the
library to handle requests for FUSE_CAP_POSIX_ACL correctly, i.e., set
the FUSE_POSIX_ACL flag in the FUSE_INIT reply to the kernel.

Signed-off-by: Marcin Sulikowski <marcin.sulikowski@editshare.com>