multiple AND-combinated `validate.pattern` clause

It's sometimes easier to provide multiple patterns for a given field rather than trying to put all several constraints into one regex.
Example: test that a value is an actual URL and contains at least a set of handful fields.

Start by tracing how the form plugin currently parses and applies validate.pattern clauses. Confirm the intended syntax for supplying multiple patterns and how existing validation errors are reported. Done means a field can require all supplied patterns to match, with coverage for the URL-and-fields example and no regression for single patterns.

Written by the indexing model from the issue text.