dd(1) argument mangling (fix by documenting)

[Ian F. Darwin]

Index:	bin/dd.c 4.2BSD 

Description:
	The man page for dd(1) is nebulous about which CONV=
	parameters can be combined. Perhaps this is an attempt
	at emulating the etymological origins of the command name,
	but the humor is lost after you waste many hours rewriting tapes.

Repeat-By:
	dd if=foo/bar of=/dev/rmt1 conv=ibm,block
	cat /dev/rmt1

	You will see that the file on tape is in ASCII,
	indicating that the `block' option has caused the `ibm'
	option to be silently ignored.

Fix:
	While `block' is new in 4.?BSD, some vagueness is present
	in V7, System V et. al, although it is unlikely to cause
	serious confusion except in 4.?BSD.

	To fix it, add the following paragraph to BUGS in the manual page:

.PP
Certain combinations of arguments to 
.I conv=
are permitted.
However, the
.I block
or
.I unblock
option cannot be combined with
.IR ascii ,
.IR ebcdic ""
or
.IR ibm .
Invalid combinations
.I "silently ignore"
all but the last mutually-exclusive keyword.
-- 
Ian Darwin, Toronto
{ihnp4|decvax}!utcs!ian