[dynui] Generalise the concept of a menu to a dynamic user interface

[dynui] Generalise the concept of a menu to a dynamic user interface

We currently have an abstract model of a dynamic menu as a list of
items, each of which has a name, a description, and assorted metadata
such as a shortcut key.  The "menu" and "item" commands construct
representations in this abstract model, and the "choose" command then
presents the items as a single-choice menu, with the selected item's
name used as the output value.

This same abstraction may be used to model a dynamic form as a list of
editable items, each of which has a corresponding setting name, an
optional description label, and assorted metadata such as a shortcut
key.  By defining a "form" command as an alias for the "menu" command,
we could construct and present forms using commands such as:

  #!ipxe
  form                     Login to ${url}
  item          username   Username or email address
  item --secret password   Password
  present

or

  #!ipxe
  form                Configure IPv4 networking for ${netX/ifname}
  item netX/ip        IPv4 address
  item netX/netmask   Subnet mask
  item netX/gateway   Gateway address
  item netX/dns       DNS server address
  present

Reusing the same abstract model for both menus and forms allows us to
minimise the increase in code size, since the implementation of the
"form" and "item" commands is essentially zero-cost.

Rename everything within the abstract data model from "menu" to
"dynamic user interface" to reflect this generalisation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>