make on/off/unplugged work like mod_pagespeed

[jeffkaufman]

In mod_pagespeed there are three settings:

• ModPagespeed on: handle requests, optimize, etc/
• ModPagespeed off: allow PageSpeed to be turned on by a nested configuration or query params, handle requests for .pagespeed. resources if they come in, mostly don't do anything.
• ModPagespeed unplugged: really don't do anything at all, including not allow turning on in nested configs.

In ngx_pagespeed "on" means "on" and "off" means approximately "unplugged". This is confusing, and means you can't do something like:

server {
   pagespeed off;
   location /foo {
      pagespeed on;
   }
}

We should make mod_pagespeed and ngx_pagespeed more similar here, probably by making ngx_pagespeed act like mod_pagespeed.

[jeffkaufman]

This can't go into a patch release for 1.9, but probably could go into 1.10.

[jeffkaufman]

What I'm thinking of doing now:

• split "off" into "unplugged" and "standby"
• "unplugged" means what it does in apache (and what "off" means in nginx)
• "standby" means what "off" does in apache
• deprecate "off"

One hitch is that it looks like ngx_pagespeed currently segfaults if you set "unplugged". Looking into that.

[jeffkaufman]

Splitting the segfault off into #1361

[jeffkaufman]

Changes proposed in #968 (comment) are now done in apache/incubator-pagespeed-mod#1482 and #1365