Add large block support to zpios(1) benchmark

Add large block support to zpios(1) benchmark

As part of the large block support effort, it makes sense to add
support for large blocks to **zpios(1)**. The specifying of a zfs
block size for zpios is optional and will default to 128K if the
block size is not specified.

  `zpios ... -S size | --blocksize size ...`

This will use *size* ZFS blocks for each test, specified as a comma
delimited list with an optional unit suffix. The supported range is
powers of two from 128K through 16M. A range of block sizes can be
tested as follows: `-S 128K,256K,512K,1M`

Example run below
(non realistic results from a VM and output abbreviated for space)

```
 --regioncount=750 --regionsize=8M --chunksize=1M --offset=4K
 --threaddelay=0 --cleanup --human-readable --verbose --cleanup
 --blocksize=128K,256K,512K,1M

 th-cnt  rg-cnt  rg-sz  ch-sz  blksz  wr-data wr-bw   rd-data rd-bw
---------------------------------------------------------------------
 4       750     8m     1m     128k   5g      90.06m  5g      93.37m
 4       750     8m     1m     256k   5g      79.71m  5g      99.81m
 4       750     8m     1m     512k   5g      42.20m  5g      93.14m
 4       750     8m     1m     1m     5g      35.51m  5g      89.36m
 8       750     8m     1m     128k   5g      85.49m  5g      90.81m
 8       750     8m     1m     256k   5g      61.42m  5g      99.24m
 8       750     8m     1m     512k   5g      49.09m  5g     108.78m
 16      750     8m     1m     128k   5g      86.28m  5g      88.73m
 16      750     8m     1m     256k   5g      64.34m  5g      93.47m
 16      750     8m     1m     512k   5g      68.84m  5g     124.47m
 16      750     8m     1m     1m     5g      53.97m  5g      97.20m
---------------------------------------------------------------------
```

Signed-off-by: Don Brady <don.brady@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3795
Closes #2071