This commit improves the emulation of recvfrom/sendfrom,
recvmsg/sendmsg, and recvmmsg/sendmmsg system calls. In particular,
MSG_DONTWAIT flag is allowed though not really emulated (benign in
most cases). Also, it is possible now to send/receive FDs via
SCM_RIGHTS on a UNIX domain socket (only send/recv of pipes and UNIX
domain sockets is currently supported). Corresponding LibOS test
is added.