Some minor improvements to read_msg_fd() made it safe to use both

Some minor improvements to read_msg_fd() made it safe to use both
set_msg_fd_in() and read_msg_fd() during the early phase of being
a client sender (up through the sending of the file list).  This
makes sure that the sender monitors the socket from the receiver
for any messages that it might send to us, and thus avoids a hang
when verbosity is high.