updated for version 7.3.970
Problem:    Syntax highlighting can be slow.
Solution:   Include the NFA regexp engine.  Add the 'regexpengine' option to
	    select which one is used. (various authors, including Ken Takata,
	    Andrei Aiordachioaie, Russ Cox, Xiaozhou Liua, Ian Young)
Added tag v7-3-970 for changeset ccecb03e5e8b
Merge v7-3-970