Fix sending fd through sendmsg/recvmsg ancillary data

ltt-kconsumerd was only reading a "char" rather than an integer. It
worked fine on little endian for values below 256, and failed on big
endian machines (always reading "0").

Also cleanup msg initialization.
Also perform final msg.msg_controllen = cmsg->cmsg_len; after memcpy, as
done in cmsg(3) example (extra safety).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>