sysusers: fix selinux context of backup files

Also, fix fopen_temporary_label to set proper context. By chance,
all users so far used the same context, so the error didn't matter.

Also, check return value from label_init().

https://bugzilla.redhat.com/show_bug.cgi?id=1121806
update-done: set proper selinux context for .updated

https://bugzilla.redhat.com/show_bug.cgi?id=1121806
Add IFLA_VTI defines to missing.h
configure: add -Wno-typedef-redefinition

Message-ID: <CAJ+BJsz9Qy-JU25dUuGT2n0LMNzcOpoJNGNtbf_boSuT_6hONw@mail.gmail.com>
Merge systemd-verify with systemd-analyze
shell-completion: systemd-analyze verify, systemctl link

Some zsh completion helpers were not installed, so completion
was broken.

Add systemd-analyze verify. Make systemctl link complete only
unit names.
man: merge systemd-verify with systemd-analyze