shell: telnet: Don't close the connection on EAGAIN error

EAGAIN error is returned if the tcp window size is full. Retry
sending the packet instead of closing the connection if this
error occurs.

Also the full payload may not be sent in a single call to
net_context_send(). Keep track of the number of bytes remaining
and try to send the full payload.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>