Linux console: don't translate ESC TAB to `backtab' in input-decode-map.

Linux console: don't translate ESC TAB to `backtab' in input-decode-map.

This translation happened after the terminfo entry for <shift>TAB in the linux
section was changed to kcbt=\E^I in ncurses version 6.3.

* lisp/term/linux.el (terminal-init-linux): Add a define-key form to remove
the entry for "\e\t" from input-decode-map.

* etc/PROBLEMS: Add a new section under "character terminals" about S-TAB
wrongly doing the same thing as M-TAB, giving tips about amending the Linux
keyboard layout.