We have the "internal" DHCP plugin. That's our preferred plugin,
and eventually we may drop all other plugins.

Currently, the "internal" plugin is based on code from systemd-networkd
and implemented in "src/dhcp/nm-dhcp-systemd.c". As this code is forked
we eventually want to switch to nettools' n-dhcp4 library (for IPv4).
For that reason we already have "src/dhcp/nm-dhcp-nettools.c".

Note that "nettools" can be configured as a DHCP plugin, but this configuration
is only experimental and for testing. There is never supposed to be a
"nettools" plugin, but eventually the "internal" plugin will switch
implementation.

We don't want to replace systemd-based implementation right away. Not until
we are sure that nettools works well. For that reason we keep them
both in parallel for a while.

This commit makes "systemd" DHCP plugin explicitly configurable
in NetworkManager.conf. Like "nettools" this is an undocumented option,
only for testing.

If you choose "internal" (the default), you get one of the
implementations (currently the "systemd" one). But by selecting
"systemd" or "nettools" explicitly, you can select the exact plugin.