HTTP: added r.readRequestForm().

HTTP: added r.readRequestForm().

The async method parses the client request body as an HTML form
and returns a Promise resolving to a form object with get(),
getAll(), has(), forEach(), hasFiles() accessors.

Supports "application/x-www-form-urlencoded" and "multipart/form-data"
content types.  File parts are detected but their contents are not
exposed.  An optional maxKeys option caps the number of fields.

File parts are detected but their contents are not exposed.  A
proper File API with streaming Blob semantics is a significant amount
of work and is out of scope.