bug#59150: 28.1; Comint shell silently send ^L

[LdBeth <andpuke@foxmail.com>]

To produce this bug, set login shell to ksh93u+ (which is the default
ksh on macOS and available on some package managers), I'm not sure if
other versions of ksh can reproduce this.

However I believe this might also affect all other command line
programs that does not translate Control L as "refresh" command.

Also I have only tried the Emacs GUI on macOS.

Procedures to reproduce the issue:

1. Start M-x shell, resize the window by dragging mouse several times

2. then several "> " occurs:

$ > > > > > > > > >

3. then type a command like "ls", and press enter key

4. the shell would respond:

^L^L^L^L^Lls: not found


I guess it is somewhere in shell.el has configured the behavior so
whenever window size has changed the shell prompt is refreshed.

However this does not work out for shell programs not like bash, and
the popup minibuffer like `ivy' or `which-key' would also cause
sending "^L".

This would annoying when have the shell opened as a split window while
editing files, although I believe set default shell to bash
is a workaround, I think this is still a bug of Emacs and worth to be
reported.

ldbeth