etc/NetworkManager: move relevant parts to conf.d/

I have no idea when I have previously looked into those two files (git
history would probably tell me), but I don't think they make much sense,
while the important parts can be cut into conf.d/ and applied
individually as needed.

etc/NetworkManager/NetworkManager.conf

@@ -1,28 +0,0 @@
-## I am using this file supporting memory, I don't copy-paste this
-## directly to anywhere.
-
-[main]
-# Ubuntu defaults
-plugins=ifupdown,keyfile,ofono
-# Use dnssec-triggerd (or dnsmasq at home)
-dns=unbound
-
-[keyfile]
-# Disable misbehaving NIC
-unmanaged-devices=mac:xx:xx:xx:xx:xx:xx
-#hostname=HOSTNAME_TO_SEND_TO_DHCPD
-
-[ifupdown]
-# more Ubuntu defaults
-managed=false
-
-## HTTP-based connectivity check
-[connectivity]
-# On Arch I seem to be using the default instead.
-uri=http://koti.kapsi.fi/~mikaela/check_network_status.txt
-interval=300
-
-# Defaults for all connections
-[connection]
-# Enable IPv6 privacy extensions, but this doesn't seem to work.
-ipv6.ip6-privacy=2

etc/NetworkManager/NetworkManager.conf.arch

@@ -1,12 +0,0 @@
-[main]
-plugins=keyfile
-dns=dnsmasq
-
-## HTTP-based connectivity check
-[connectivity]
-uri=http://nmcheck.gnome.org/check_network_status.txt
-interval=100
-
-[keyfile]
-#unmanaged-devices=mac:xx:xx:xx:xx:xx:xx
-#hostname=HOSTNAME_FOR_DHCPD

etc/NetworkManager/conf.d/connectivity.conf

@@ -0,0 +1,10 @@
+[connectivity]
+uri=http://nmcheck.gnome.org/check_network_status.txt
+interval=100
+
+# Expect "NetworkManager is online" to check for captive portals.
+
+# Debian: don't bother with this file,
+# sudo apt install network-manager-config-connectivity-debian
+# for /usr/lib/NetworkManager/conf.d/20-connectivity-debian.conf
+# (http://network-test.debian.org/nm without interval)

etc/NetworkManager/conf.d/ipv6privacy.conf

@@ -0,0 +1,4 @@
+# Defaults for all connections
+[connection]
+# Enable IPv6 privacy extensions, but this doesn't seem to work.
+ipv6.ip6-privacy=2

etc/NetworkManager/conf.d/keyfile.conf

@@ -0,0 +1,4 @@
+#[keyfile]
+# Disable misbehaving NIC
+#unmanaged-devices=mac:xx:xx:xx:xx:xx:xx
+#hostname=HOSTNAME_TO_SEND_TO_DHCPD