hterm: implement Character Position Relative (HPR)
hterm: implement Character Position Relative (HPR)

Simple enough sequence to implement, and helps fill out the existing
set of functions to control the cursor position we already support.
Behavior compared against xterm and they match.

Change-Id: I0d313715474096ae4c1798f89a4dbbb5f52e0232
Reviewed-on: https://chromium-review.googlesource.com/493126
Reviewed-by: Brandon Gilmore <adbfbaf7e4f927b45ce8ad3e5043a632414a833b@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm