Add the notion of the settings hierarchy, complete with
register/unregister routines.
Rename set->store and get->fetch to avoid naming conflicts with get/put
as used in reference counting.
Add the concept of an abstract configuration setting, comprising a (DHCP)
tag value and an associated byte sequence.
Add the concept of a settings namespace.
Add functions for extracting string, IPv4 address, and signed and
unsigned integer values from configuration settings (analogous to
dhcp_snprintf(), dhcp_ipv4_option(), etc.).
Update functions for parsing and formatting named/typed options to work
with new settings API.
Update NVO commands and config UI to use new settings API.
show_setting() and related functions now return an "actual length" in the
style of snprintf(). This is to allow consumers to allocate buffers large
enough to hold the formatted setting.
Introduce new {show,set,clear}_setting() that take a struct setting *
rather than a const char *.
set_setting() handles calling clear_setting() when appropriate, so that
individual setting types don't have to check for empty strings.
