autogen: drop unnecessary -Wno-gnu for the clang shortcut

It is no longer necessary after 65e3a2cf7c3b399853dd309f702ca5078b7d16ea
where -Wno-gnu-variable-sized-type-not-at-end is added if we detect
that the compiler is clang.
pam_systemd: remove unused null check

username was already checked with isempty() and cannot be null at
this point.

CID#1237766
udev: silence TOCTOU warning when creating a directory

CID#979416. There is no real race here to fix, but lets make coverity
happy and rework the code.

Note that we still fail if the directory is removed _after_ we ran
mkdir(), so the same race is still there. Coverity is complaining, though.
Rewrite the code to make it happy.

(David: rewrote the commit-message to note that this is not a race. If I'm
        wrong, blame me, not Ronny!)
log: unchecked return value from library

fix 1237557 Unchecked return value from library
resolve: fix CID#1237549 Unchecked return value
bus-socket: fix CID#996290 Unchecked return value
namespace: unchecked return value from library

fix:

CID 1237553 (#1 of 6): Unchecked return value from library
(CHECKED_RETURN

CID 1237553 (#3 of 6): Unchecked return value from library
(CHECKED_RETURN)

CID 1237553 (#4 of 6): Unchecked return value from library
(CHECKED_RETURN)

CID 1237553 (#5 of 6): Unchecked return value from library
(CHECKED_RETURN

CID 1237553 (#6 of 6): Unchecked return value from library
(CHECKED_RETURN)
sd-dhcp-client: fix REBOOT state handling

client->secs wasn't getting set in the REBOOT state, causing
an assertion.  REBOOT should work the same way as INIT, per
RFC 2131:

 secs   2  Filled in by client, seconds elapsed since client
           began address acquisition or renewal process.

REBOOT is necessary because some DHCP servers (eg on
home routers) do not hand back the same IP address unless the
'ciaddr' field is filled with that address, which DISCOVER
cannot do per the RFCs.  This leads to multiple leases
on machine reboot or DHCP client restart.
sd-dhcp-lease: expose load/save functions

Based on patch by Dan Williams.
update TODO
sd-dhcp-client: allow getting/setting the client ID

The client identifier can be in many different formats, not just
the one that systemd creates from the Ethernet MAC address.  Non-
ethernet interfaces may have different client IDs formats.  Users
may also have custom client IDs that the wish to use to preserve
lease options delivered by servers configured with the existing
client ID.
sd-dhcp-lease: load/save client ID

The lease is usually tied to the client ID, so users of the
lease may want to know what client ID it was acquired with.
systemd-logind.service: set Type=notify

The code already calls sd_notify("READY=1"), so we may as well take
advantage of the startup behavior in the unit. The same was done for
the journal in a87a38c20.
busctl: split out introspection parser from tree logic so that we can reuse it for a future "busctl introspect" command
set: make set_consume() actually free the allocated string if the string already is in the set
tests: fix minor memory leak
core: watchdog bus properties cannot be both writable and constant
sd-bus: refuse properties that claim to be both writable and constant at the same time
busctl: add new "introspect" verb for introspecting objects
busctl: improve output of service creds