[9fans] New internal command for acme proposal (with implementations)

[Uvelichitel <uvelichitel@gmail.com>]

	Hi 9fans. While working with large sources in acme I feeling some lack
of navigation capacity. So two new cmd. 1) 'Rev' , works precisely as
'Look' but in reverse direction. It also change behavior of right mouse
button search to search reverse. You will notice change with pointer
position and little symbol near pipe in tag. Executing 'Look' instantly
change behavior back. After all navigation 2) 'Tip' , return pointer to
last typing point. It understand 2-1 chording, so you can handy insert
selection to last insert point without snarfing, including same window.
	I can't implement cmd as external, so I did little changes in original
acme sources. I did all my best to make it accurate. Things work for me
on p9p for around a year. You can try it at
https://bitbucket.org/uvelichitel/acme. Repo contains full src/cmd/acme
dir, which you can 'mk install' and patch.diff to see changes. Any
feedback very appreciated, proposal can be implemented. In positive
case I can try to commite to p9p official repo at least.
	Best regards.