build: fix wrongly linking against libreadline in all applications

Every Makefile in the subtrees would include -lreadline
as part of LIBS, hence every application would link against
the library.

This was broken since we added 'm4/ax_lib_readline.m4'.

Fixes: 29297f8531e24d4df616c5c3acace9e23a695a02
libnm: (trivial) correct an example in NMSecretAgentOldGetSecretsFunc docs
vpn: don't choose route priority for VPN connections based on the parent device

The parent device is just the device that happens to be the best device.
It does not mean, that its route metric should be inherited to the VPN
connection.

This also makes the resulting route metric for VPN connections much
more predictable: now it is either ipv4.route-metric from the connection,
or it falls back to NM_VPN_ROUTE_METRIC_DEFAULT (10).
core: adjust values for default route-metrics per device type

We recently changed default values for route metrics. Revise that
again and increase the space between the default values.

No strong reason to do this, but it seems better to have larger
gaps and make use of the available range.
devices: NM_SETTING_WIRED_S390_SUBCHANNELS is now G_TYPE_STRV (rh #1168764)

Fixes crash on s390.
The type change of the property was done in 6a4127cfa063a77e3ee886a999441c71f6a0e11c,
but we forgot to update this function.

https://bugzilla.redhat.com/show_bug.cgi?id=1168764
cli: fix setting ethernet.s390-subchannels

It was not properly updated in a7c4d53d036c0f75b0903c95de7cb8fbdc47413f (when
updating stuff to ne libnm).

And allow 2 subchannels (libnm and ifcfg-rh allow it too).
keyfile: assert in writer to have valid connection id

Few lines above we already verify the connection. We must
have an @id.
nmcli/bash-completion: complete [+-] modifier for `connection modify` property

Fixes
  nmcli connection modify CONNECTION +conn<TAB>