[gdb/tui] Introduce translate_acs

In function tui_update_variables we have the somewhat inconvenient:
...
  entry = translate (tui_border_kind, tui_border_kind_translate_lrcorner);
  int val = (entry->value < 0) ? ACS_LRCORNER : entry->value;
...

Add a new function translate_acs, that allows us to do the more straighforward:
...
  int val = translate_acs (tui_border_kind, tui_border_kind_translate_lrcorner,
			   ACS_LRCORNER);
...

By special-casing "acs" in translate_acs, we can now remove the acs entries
from the translation tables.

Tested on x86_64-linux.

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