libmisc, man: Drop old check and advice for complex character sets in passwords

libmisc, man: Drop old check and advice for complex character sets in passwords

Add the relevant XKCD to the passwd(1) manual page.  It already explains
most of the rationale behind this patch.

Add also reference to makepasswd(1), which is a good way to generate
strong passwords.  Personally, I commonly run `makepasswd --chars 64` to
create my passwords, or 32 for passwords I need to type interactively
often.

The strength of a password is an exponential formula, where the base is
the size of the character set, and the exponent is the length of the
password.  That already shows why long passwords of just lowercase
letters are better than short Pa$sw0rdZ3.  But an even more important
point is that humans, when forced to use symbols in a password, are more
likely to do trivial substitutions on simple passwords, which doesn't
increase strength, and can instead give a false sense of strength, which
is dangerous.

Closes: <https://github.com/shadow-maint/shadow/issues/688>
Link: <https://xkcd.com/936/>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>