pw(8): fix combination of modes -N and -w random

The command "pw usermod nobody -Nw random" (or useradd)
generates random password and prints it in encrypted form
but skips choosen random string that makes not much sense
and contradicts the manual page pw.8

Fix it by showing random password in plain text with -N and
without it equally. Add yet another example of how to generate
pw-style random password.

MFC after: 2 weeks