[dynui] Add concept of a secret user interface item

[dynui] Add concept of a secret user interface item

For interactive forms, the concept of a secret value becomes
meaningful (e.g. for password fields).

Add a flag to indicate that an item represents a secret value, and
allow this flag to be set via the "--secret" option of the "item"
command.

This flag has no meaning for menu items, but is silently accepted
anyway to keep the code size minimal.

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