* Revert "tui: improve support for tmux, GNU Screen"

This reverts commit c266c2f36ea56220268d2b19364e28815e338c2f.

This commit broke neovim under tmux, where it "types" the background
string as input to the terminal. Fixing this with neovim as-is requires
changing tmux keybindings or TERM settings, which has to be done by
every single user, therefore a fix should happen on neovim's side.

Fixes #14298

* tui: fix code lints.

Closes #14573

Wrapping it in DCS allows the get_bg features to work with tmux, GNU Screen.

assert() is compiled out for release builds, but we don't want to
continue running in these impossible situations.

This also resolves the "implicit fallthrough" warnings for the asserts
in switch cases.

Adjust the test for handle_background_color() according to
bd0275182b1c1b14c43dc4fc7e9f9da05071e56c.

Terminal responses may be fragmented. In that case, the problem that was
not processed normally and was processed in the same way as user input
is corrected.

fixes #11393.

fixes #11699, #11991.

termkey_strfkey() formats ctrl-l and ctrl-shift-l as <C-l> and <C-L>,
respectively.  Nvim wants the latter to look like <C-S-L>, since <C-l>
and <C-L> are interpreted the same way.

This is only required when the Ctrl modifier is present.

This allows us to remove special-case handling of Esc in
forward_modified_utf8(), which was always sending "<Esc>" to nvim even
when there were modifiers present.

Closes #12584