Highlight non-existent commands in M-x shell

Highlight non-existent commands in M-x shell

* lisp/shell.el (shell-mode): Enable highlighting of non-existent
commands if requested.
(shell-highlight-undef-aliases):
(shell-highlight-undef-remote-file-name-inhibit-cache): New user
options.
(shell-highlight-undef-mode): New minor mode.
(shell-highlight-undef-defined-face):
(shell-highlight-undef-undefined-face):
(shell-highlight-undef-alias-face): New faces.
(shell-highlight-undef--exec-cache):
(shell-highlight-undef--face):
(shell-highlight-undef-keywords):
(shell-highlight-undef-regexp):
(shell-highlight-undef--executable-find):
(shell-highlight-undef-matcher):
(shell-highlight-undef--indirect):
(shell-highlight--setup):
(shell-highlight-undef-reset-mode): New functions and buffer local
variables (bug#51940).