incoming: don't handle 0xFFFFFFFF timestamps special in n_dhcp4_incoming_query_u32()

First of all, from the naming of n_dhcp4_incoming_query_u32() it is
confusing to coerce 0xFFFFFFFF to zero. It should just return the
plain value.

Also note that n_dhcp4_incoming_query_u32() only has three callers:
n_dhcp4_incoming_query_lifetime(), n_dhcp4_incoming_query_t1() and
n_dhcp4_incoming_query_t2().

Looking further, those three functions only have one caller:
n_dhcp4_incoming_get_timeouts(). Note how the code there already tries
to handle UINT32_MAX and interprets it as infinity (UINT64_MAX).
But as it was, UINT32_MAX never actually was returned.

It seems that RFC [1] does not specially define the meanings of
0xFFFFFFFF and 0. It sounds reasonable to assume that 0 just means
0 lifetime, and 0xFFFFFFFF means infinity. On the other hand, compare
this to systemd's code [2], which coerces 0 to 1. This does not seem
right to me though. Note how systemd returns 0xFFFFFFFF as-is.

Drop the special handling of 0xFFFFFFFF from n_dhcp4_incoming_query_u32().
It now just returns the plain value and it's up to n_dhcp4_incoming_get_timeouts()
to make sense of that. This will fix behavior, so that 0xFFFFFFFF will be
reported as infinity, and not as zero.

[1] https://tools.ietf.org/html/rfc2132#section-9.2
[2] 68c2b5ddb1/src/libsystemd-network/sd-dhcp-lease.c (L553)

shared/n-dhcp4/src/n-dhcp4-incoming.c

@@ -365,10 +365,7 @@ static int n_dhcp4_incoming_query_u32(NDhcp4Incoming *message, uint8_t option, u
 
         memcpy(&be32, data, sizeof(be32));
 
-        if (be32 == (uint32_t)-1)
-                *u32p = 0;
-        else
-                *u32p = ntohl(be32);
+        *u32p = ntohl(be32);
         return 0;
 }