sd-dhcp-client: include client id in debug messages
networkd: guard against NULL pointers

This should never be a problem, but better be safe than sorry.
Document CONFIG_NET_NS as a required kernel option

Several units now utilize the PrivateNetwork parameter, which requires
network namespace support.
login: add 'mir' to the list of session types

Add Mir to the list of session types. This is implemented for LightDM
in lp:~robert-ancell/lightdm/xdg-session-desktop [1].

[1] https://code.launchpad.net/~robert-ancell/lightdm/xdg-session-desktop/+merge/214108

(david: adjusted commit-header and fixed whitespace issues)