odhcpd: struct lease -> struct lease_cfg

odhcpd: struct lease -> struct lease_cfg

Both the DHCPv4 and DHCPv6 RFCs use the term "lease" to refer to what in the
odhcpd codebase is "struct assignment".

Some examples:

RFC2131, §2.2:

   In this document, the period over which a network address
   is allocated to a client is referred to as a "lease" [11].  The
   client may extend its lease with subsequent requests.  The client may
   issue a message to release the address back to the server when the
   client no longer needs the address.  The client may ask for a
   permanent assignment by asking for an infinite lease.  Even when
   assigning "permanent" addresses, a server may choose to give out
   lengthy but non-infinite leases to allow detection of the fact that
   the client has been retired.

RFC8415, §4.2:

   lease                     A contract by which the server grants the
                             use of an address or delegated prefix to
                             the client for a specified period of time.

For example in §7.3:

   RENEW (5)                 A client sends a Renew message to the
                             server that originally provided the
                             client's leases and configuration
                             parameters to extend the lifetimes on the
                             leases assigned to the client and to update
                             other configuration parameters.

There's also a bit of confusion in the odhcpd codebase, where the two are
sometimes used interchangeably, see e.g. handle_*_leases() in ubus.c which
correspond to the ubus methods "ipv4leases" and "ipv6leases" but which then
returns "assignments", or dhcpv4_lease() in dhcpv4.c, which returns an
"assignment". There's more examples.

To put it differently, "assignment" when used in the RFCs is essentially
the verb, the server assigns, and "lease" is the noun - the server
assigns a lease.

So rename "struct lease" to "struct lease_cfg" to better describe what it is
used for. I.e. it contains the cfg for a host which can provide the parameters
for a static lease (and it comes from a "host" uci section) but it isn't a
*lease* per se.

This is a bit a of pet peeve of mine, but I found it quite confusing when I
first started hacking on odhcpd. It also lays the foundation for later patches
which rework/split/rename "struct assignment".

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/292
Signed-off-by: Álvaro Fernández Rojas <[email protected]>