units: order systemd-tmpfiles-clean.service after time-sync.target

That way, on systems lacking an RTC we don't false start removing aged
files too early.
copy: don't eat up error from chown()/chmod()

The idea was to not fail on, nor to ignore errors from chown()/chmod(),
but to proceed and simply return the most recent error...
sd-dhcp6-client: remove bogus dependency on dhcp6-icmp6.h

Makefile.am had a reference to it but it none of the sources included it.

Tested: "make dist" works again after this patchset is applied.

Fixes: 2ea8857effb833615b16d10fc7a19a7104c19e13
sd-dhcp6-client: fix path of sd-icmp6-nd.h in Makefile.am

It was incorrectly looking for a file in src/libsystemd-network/ when the file was actually deployed to src/systemd/ instead.  This broke "make dist".

Tested: "make dist" works again after this patchset is applied.

Fixes: f20a35cc0d537dd4cfc1054cf7936b04a1700f3a
sd-dhcp6-client: include dhcp6-protocol.h in the list of sources

If the file is not listed, then "make dist" will not include it.

Tested: "make distcheck" works after this fix is applied.

Fixes: 139b011ab81ccea1d51f09e0261a1c390115c6ff
coredump: fix include of xattr.h

The correct path is now <sys/xattr.h> (from glibc-headers) and no longer
<attr/xattr.h> (from libattr-devel.)

Fixes: 34c10968cbe3b5591b3c0ce225b8694edd9709d0