[gdb/tui] Merge tui border-kind corner translation tables

The tables:
- tui_border_kind_translate_ulcorner
- tui_border_kind_translate_urcorner
- tui_border_kind_translate_llcorner
- tui_border_kind_translate_lrcorner
are identical.

Merge and rename to tui_border_kind_translate_corner.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>