The caller should make a conscious decision which delimiters to use.
Unfortunately, there is a variety of different demiters in use. This
should be unitfied and the callers should use one of a few specific
set of delimiters.

This could be unified by (re)using a define as delimiters, like

   strv = nm_utils_strsplit_set_full (value, MULTILIST_WITH_ESCAPE_CHARS, NM_UTILS_STRSPLIT_SET_FLAGS_ALLOW_ESCAPING);

where MULTILIST_WITH_ESCAPE_CHARS has a particular meaning that should
be reused for similar uses.

However, leaving the delimiter at NULL is not good because it's unclear who
wants that default behavior (and what the default should be). Don't allow that.

There are almost no callers that relied on this default anyway.

It will be useful to extend nm_utils_strsplit_set() with various
flavors and subtly different behaviors. Add a flags argument to
support these.

Fixes: 6aa66426a416 ('settings/ifupdown: merge eni_ifaces and connections hashes in plugin')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/145
https://bugzilla.redhat.com/show_bug.cgi?id=1694912

Fixes a crash on failed AddAndActivate:

  $ ip link set eth0 down
  $ nmcli d conn eth0
  Error: Failed to add/activate new connection: Connection 'eth0' is not available on device eth0 because device has no carrier
  <NetworkManager crashes>

  #3  0x000055555558b6c5 in _nm_g_return_if_fail_warning
  #4  0x00005555557008c7 in nm_settings_has_connection
  #5  0x0000555555700e5f in pk_add_cb
  #6  0x0000555555726e30 in pk_call_cb
  #7  0x0000555555726e30 in pk_call_cb
  #8  0x0000555555726e30 in pk_call_cb
  #9  0x00005555555aaea8 in _call_id_invoke_callback
  #10 0x00005555555ab2e8 in _call_on_idle

https://github.com/NetworkManager/NetworkManager/pull/325

initscripts support rule-* and rule6-* files for that.

Up until now, we ignored these files for the most part, except if
a user configured such files, the profile could not contain any static
routes (or specify a route-table setting). This also worked together
with the dispatcher script "examples/dispatcher/10-ifcfg-rh-routes.sh".

We cannot now start taking over that file format for rules. It might
break existing setups, because we can never fully understand all rules as
they are understood by iproute2. Also, if a user has a rule/rule6 file and
uses NetworkManager successfully today, then clearly there is a script
in place to make that work. We must not break that when adding rules
support.

Hence, store routing rules as numbered "ROUTING_RULE_#" and
"ROUTING_RULE6_#" keys.

Note that we use different keys for IPv4 and IPv6. The main reason is
that the string format is mostly compatible with iproute2. That means,
you can take the value and pass it to `ip rule add`.
However, `ip rule add` only accepts IPv4 rules. For IPv6 rules, the user
needs to call `ip -6 rule add`. If we would use the same key for IPv4
and IPv6, then it would be hard to write a script to do this.
Also, nm_ip_routing_rule_from_string() does take the address family as
hint in this case. This makes

  ROUTING_RULE_1="pref 1"
  ROUTING_RULE6_1="pref 1"

automatically determine that address families. Otherwise, such
abbreviated forms would be not valid.