[tc_terminal] Fix `tcterm_get_cursor_pos()`

[darkuranium]

The tcterm_get_cursor_pos() function currently flushes all input in order to get the data in POSIX — that should be fixed.

The best way to do that is to take advantage of buffering (depends on #12 being implemented), then do a flush and fetch position. The CREAD flag might perhaps be helpful here.