Add support for MAGTAPE protocol.

[larsbrinkhoff]

@ams wrote:

;; Our RFC packet (to contact name MAGTAPE) contains
;; MAGTAPE r devname
;; r = "R" or "W" saying whether to open for read or for read/write.
;; devname = magtape device name.

;; Following that, we get packets with opcode 200 with data to write on tape,
;; or 201 containing commands.

;; 200 is simple: the data in the packet is written.

;; The data is written into records of a fixed size, regardless of packet boundaries.
;; The record size is specified by the set record size command.
;; You can write a smaller record by sending as much data as you want,
;; followed by a force-output command.

;; We do not reply to data packets.
;; Every so often, you should send an output-status inquiry command
;; to find out whether the output data got an error (such as end of tape).
;; All data packets past the error are ignored, and the reply to the
;; output-status command tells you how many packets were handled (not ignored)
;; which will tell you where the error occurred.

;; The error code returned by the output-status inquiry describes the
;; success, or reason for failure, of the output data packets.
;; If the error code is %ereot (end of tape), then the data in the packet
;; that got the error was all written properly, but following packets were ignored.
;; You should send a couple of write tape mark commands to finish the tape.

;; 201 packets are formatted as follows:

;; Four 8-bit bytes, the first of which contains a command code
;; and the next three of which are a transaction code.

;; Arguments to the command occupy four bytes each,
;; and they are turned into numbers by using the earlier bytes
;; as the less significant bits of the number.