captcha refresh

Optional global config to pull in that creates the
[captcha-refresh] tag to ask for a new captcha image
if it can't be read.
IC cache busting for captcha refresh.
Suppress error message in [assign] when using the documented behaviour of passing an empty string.
Fix logging messages for Jobs so ErrorDestination can redirect them

Allows directives like this to work:

ErrorDestination  "Run jobs group=%s pid=%s"  logs/jobs_run.log
ErrorDestination  "Finished jobs group=%s pid=%s"  logs/jobs_run.log
Set MV_PASSWORD cookie to be secure if set from a secure page

Adds new option for UserDB, "secure_cookies". Defaults to 1 if called from HTTPS page.
Add simple tag to generate user-friendly passwords

Takes no options at all.

Could profitably have options to control length, characters used,
etc., but this has been useful to some of us for years and is simple
and has no external dependencies, so might as well get it in here.
New UserDB option for read-only attribute

* Identifies fields for which the session value will not overwrite
  the database value. Useful for protected fields that are only to
  be managed by site administrators, particularly for outboard fields
  where you don't want [userdb] to try to create an outboard record
  at all.

* Syntax similar to UserDB 'scratch' attribute, but only valid for keys:

  UserDb default  read_only  'field1, field2, field3'
Merge remote-tracking branch 'upstream/master'