File-local-variable support for sh-script; add mksh support

File-local-variable support for sh-script; add mksh support

* lisp/files.el (interpreter-mode-alist): Add mksh.

* lisp/progmodes/sh-script.el (sh-ancestor-alist): Add mksh, a pdksh
derivative.
(sh-alias-alist): Alias /system/bin/sh (Android's system shell) to
mksh. Improve custom spec; allow regular expressions.
(sh-shell): Delegate name splitting to `sh-canonicalize-shell'.
(sh-after-hack-local-variables): New function.
(sh-mode): Use it; respect file-local `sh-shell' variable.
(sh-set-shell): Use `sh-canonicalize-shell' instead of open-coding
the normalization.
(sh-canonicalize-shell): Rewrite to support regexes.

Fixes: debbugs:17333