su: consistent angular brackets and lowercase in help text of 'su'

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
lib/colors: add support for auto, always and never modes

 ... to implement --color[=<when>]

Signed-off-by: Karel Zak <kzak@redhat.com>
dmesg: support --color[={auto,always,never}]

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