With this, parsing the properties address/route (for both IPv4/IPv6)
has a runtime complexity of O(n*ln(n)).

Previously, parsing these properties was O(1), but the constant factor
was very high because for each address/route x ipv4/ipv6 combination we would
search about 2*1001 times whether there is a matching value.
Now the runtime complexity is O(n*ln(n)) for each of these 4 properties
where n is the number of entries in the keyfile.

Also note, that we only have 4 properties for which the parsing has
this complexity. Hence, parsing the entire keyfile is still O(n) + 4*O(n*ln(n))
which reduces to O(n*ln(n)). So, parsing the entire keyfile is still benign
and the logarithmic factor comes merely from sorting (which is fast).

Now, the number of supported addresses/routes is no longer limited
to 1000 (as before). Now we would accept all keys up from 0 up to
G_MAXINT32.

Like before, indexes will be automatically adjusted and gaps in the
numbering are accepted. That is convenient, if the user edits the
keyfile manually and deletes some lines. And we anyway must not change
behavior.

  $ multitime -n 200 -s 0 -q ./src/settings/plugins/keyfile/tests/test-keyfile
  # build with -O2 --without-more-asserts
  # before:
                Mean                Std.Dev.    Min         Median      Max
    real        0.290+/-0.0000      0.013       0.275       0.289       0.418
    user        0.284+/-0.0000      0.010       0.267       0.284       0.331
  # after:
                Mean                Std.Dev.    Min         Median      Max
    real        0.101+/-0.0000      0.002       0.099       0.100       0.118
    user        0.096+/-0.0000      0.003       0.091       0.096       0.113
    sys         0.004+/-0.0000      0.002       0.001       0.004       0.009