[settings] Allow for multiple definitions of each predefined setting

[settings] Allow for multiple definitions of each predefined setting

Allow for multiple setting definitions with the same name but
different scopes and tags.  For example, allow for a "filename"
setting with default scope and tag value 67 (for DHCPv4) and a
corresponding "filename" setting with IPv6 scope and tag value 59 (for
DHCPv6).

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