Make new ctrl-l logic more portable

The behavior of the new scrollback-push is not consistent across various
terminals and the functionality of ctrl-l is too important to break in the
default bindings. This hack should preserve the benefits of scrollback-push
while keeping the portability of the old clear-screen: first we ask the terminal
to scroll everything offscreen *then* we clear the screen. The addition of
clear-screen should be a no-op on any terminal that fully supports the desired
behavior of invoking the new scrollback-push.

(There are no real performance-related concerns with ctrl-l.)

This handles multiple possible issues (observed or otherwise) with scrollback-push:
* The screen contents are pushed back into scrollback but the screen isn't cleared
  (observed under Prompt/iOS, FreeBSD console session)
* termcap SF isn't supported
* termcap UP isn't supported ("Only a few terminal descriptions provide these
  commands, and most programs do not use them.")

It still breaks in an ugly way if terminal position reporting isn't supported,
but I'll address that separately.

share/functions/__fish_shared_key_bindings.fish

@@ -66,7 +66,7 @@ function __fish_shared_key_bindings -d "Bindings shared between emacs and vi mod
     bind --preset $argv alt-l __fish_list_current_token
     bind --preset $argv alt-o __fish_preview_current_file
     bind --preset $argv alt-w __fish_whatis_current_token
-    bind --preset $argv ctrl-l scrollback-push repaint
+    bind --preset $argv ctrl-l scrollback-push clear-screen
     bind --preset $argv ctrl-c cancel-commandline
     bind --preset $argv ctrl-u backward-kill-line
     bind --preset $argv ctrl-w backward-kill-path-component