Don't modify argv in dd due to ',' in arguments.

Don't modify argv in dd due to ',' in arguments.

* src/dd.c: Include quotearg.h.
(operand_matches): New function.
(parse_symbols, operand_is): Use it.
(parse_symbols): 1st arg is now const pointer.  Don't modify it.
msgid arg is now just the message, not a format.
(scanargs): Add some 'const's to check for problems like the above.