Added a very simple query language for name or attribute based lookups.

This is a language suitable for simple queries using the lookup form. It
supports quoted and unquoted strings representing either regular expression
for object names or a colon-separated key/value pair for matching attribute
values.
Use the 'page' type in the error handler as well.
Don't return an HTTP error status if a handler fails.

This might confuse browsers and, more importantly, now we re-populate the
query input field with the user's query in case it failed.
Added support for looking up other object types besides hosts.

A query may now be prefixed with <type>: to chose a different type. The
default is still hosts.
Remove query data from the URI path.
Use the current username by default.