Fix the short option for 'no-warning-for-unkeyed-lists'

Dash "-" is not allowed in the short option string. Replace it
with 'K', a single character option consistent with others.

Having two options to control warnings on the unkeyed list,
although mutually exclusive, makes the default behavior unclear
when neither option is used.

Simplify the options by removing 'warning-for-unkeyed-lists'
while preserving the original default behavior of warning against
unkeyed lists when neither option is used.

JIRA: YANGTOOLS-1429
Change-Id: I6c37892ae7e39864420a0e6c15dd2fc1cf0ba952
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
(cherry picked from commit db8732c024c02cadb7814149fe23fab85533944b)