shadow-native: allow for setting password in clear text
Allow user to set password in clear text. This is convenient when
we're building out an image.
This feature is mainly used by useradd.bbclass and extrausers.bbclass.
This patch adds a new option '-P' to useradd, usermod, groupadd and groupmod
commands provided by shadow-native. The shadow package on target and in SDK
will not be affected.
