lslogins: use bit-array, cleanup SELinux code

Signed-off-by: Karel Zak <kzak@redhat.com>
lslogins: add --noheadings

That's fatal mistake that this option which is required for script
is missing.

Signed-off-by: Karel Zak <kzak@redhat.com>
lslogins: make --notruncate more robust

 - keep coldescs[] read-only
 - don't expect that only COL_GECOS uses truncate flag

Signed-off-by: Karel Zak <kzak@redhat.com>