Improve Malayalam language transliteration

Improve Malayalam language transliteration

The existing ITRANS scheme did not support some characters and
language quirks like 'chillu's.  The Inscript method had errors.
* lisp/language/ind-util.el (indian-mlm-base-table): Add archaic
chars, Mozhi combos; cleanup.
(indian-mlm-mozhi-table): New scheme Mozhi.
* lisp/leim/quail/indian.el (inscript-mlm-keytable): Correct
errors.  Add Inscript chillus & zero-width chars, Mozhi scheme.
* etc/NEWS: Mention the changes.