add "%H" to mdb_printf() for human-readable sizes
From the original Joyent bug report:
For OS-1367, we want ::umastat to emit human-readable sizes. Indeed, it's common – or should be – for dcmds to want to print human-readable sizes (e.g., like the output of the the "-h" option to ls/dh/du). It seems natural to add this as an mdb_printf() (and ::printf!) specifier. After some discussion (and some gnashing of teeth over the fact that %S has been taken for so little gain), "%H" was selected as the most appropriate specifier (and is not taken by other printf-like functionality).