MINOR: receiver: add RX_F_MUST_DUP to indicate that an rx must be duped

MINOR: receiver: add RX_F_MUST_DUP to indicate that an rx must be duped

The purpose of this new flag will be to mark that some listeners
duplicate their reference's FD instead of trying to setup a completely
new listener from scratch. This will be used when multiple groups want
to listen to the same socket, via multiple FDs.