posix-stack: fix strict aliasing violations on CMSG_DATA(cmsghdr)
posix-stack: fix strict aliasing violations on CMSG_DATA(cmsghdr)

CMSG_DATA() returns a char array which hold an object of some other
type. The current code natually calls reinterpret_cast, but that is
illegal since the compiler sees one object with two incompatible
types.

Fix by copying the raw data into a temporary object of the
expected type. The compiler should optimize it back into direct
memory access.

Fixes #752.
Message-Id: <f4885bc01796eaa1020df6dd0b9c9759f35e758d@scylladb.com>

avikivity/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar