Add hook for changing multiplicative suffix to dd operations to automati...

...cally change based upon OS and make the change for Solaris

When zxfer is called with the -D option on Solaris (I'm using OmniOS a derivative of the OpenIndiana project), the zfs send/recv fail due to dd being called with errors like:

```sh
...
Estimated size is: 563945097936
dd: invalid number ‘1m’
dd: invalid number ‘1m’
   0.0B  at    0.0B/s   eta:   0:00:00    0% [=                                                                                              ]
Copied: 0B (0.0B ) (0% of expected input)
Time:  0 seconds
Throughput: (infinite)

cannot receive: failed to read from stream
Error when zfs send/receiving.
```

The suffix for the dd commands was hardcoded to '1m' when the -D option was added.  This pull request adds the ability to automagically change this for Solaris to a '1M' but default to '1m' for other OSes.