criu --help: print a proper list of features

Remove that weird special case from check_add_feature() function, making
it a separate pr_check_features(), which prints a SEP-separated list of
feature names, obeying the max WIDTH, and prepending each line with
OFFSET. That way, we have a decept --help output:

  --feature FEAT        only check a particular feature, one of:
                            mnt_id, mem_dirty_track, aio_remap, timerfd, tun,
                            userns, fdinfo_lock, seccomp_suspend,
                            seccomp_filters, loginuid, cgroupns, autofs

Alternatively, we could just drop the functionality of showing all the
individual features to check.

[v2: use %s in pr_msg to fix a -Wformat-security warning]

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>