Exactly which escape sequences are emitted by gdb in TUI mode are
determined largely by the curses implementation. Testing my latest
(as yet unsubmitted) series to refactor the TUI showed a couple of
failures that I tracked to the test suite's terminal implementation.
In particular, the CSI "@" sequence was not implemented; and the CSI
"X" sequence was implemented incorrectly.
This patch fixes both of these problems. Tested on x86-64 Fedora 28.