mtdump dumps the record structure of a tape image file in SIMH, E11, TPC, or P7B
format.  mtdump is invoked by:

	mtdump {-lsecp} file1 file2 file3...

...where -l specifies that dumping should stop at the first double-EOF mark or
EOM mark, -s specifies standard SIMH format, -e specifies E11 format, -p
specifies P7B format, and -c specifies TPC format.  The default is to dump the
entire file through the physical EOF in extended SIMH format.  Output is to
stdout, unless redirected.

SIMH standard format is as follows:

    object
    object
    :

and so on, where "objects" are either 4-byte metadata markers or data records of
the form:

	4 byte record length marker
	data bytes padded to even length if needed
	repeat of 4 byte record length marker

The format defines three hexadecimal marker values:

    00000000 = tape mark
    FFFFFFFE = erase gap
    FFFFFFFF = end of medium

SIMH extended format is a superset of standard format that divides the metadata
and length markers into two fields: a control class consisting of the upper four
bits, and a marker-specific value or data-length value in the lower 28 bits.
The control class values are further divided into private classes and
SIMH-reserved classes.

E11 format is similar to SIMH standard format, but odd-length data records are
not padded to even lengths, and erase gap markers are not supported.

TPC format is as follows:

	2 byte record length 1
	record 1
	2 byte record length 2
	record 2
	:
	2 bytes = 0000 for end of file

and so on.  Records are padded to even byte lengths.

P7B format is confined to 7 track tapes and is as follows:

	start of record + first 6b character and parity
	:
	last character
	start of record + first 6b character and parity

and so on.  P7B data records consist 6-bit-plus-parity data bytes.  The first
record byte has the MSB set to indicate the start of the record,  The remaining
bytes omit this indicator.  A P7B tape must end with a tape mark or a dummy
"last record" indicator.  A tape mark is indicated by a character of 217 octal
(start of record, character 17, and incorrect odd parity or a reserved character
for even parity).  If a 217 tape mark is followed by additional "data" bytes of
017 octal value, the entire sequence is considered to be a single tape mark.