manual: Reorganize crypt.texi.

In preparation for a major revision of the documentation for
crypt(_r), getentropy, and getrandom, reorganize crypt.texi.  This
patch does not change any text; it only deletes and moves text.

The description of 'getpass' moves to terminal.texi, since all it does
is read a password from the controlling terminal with echo disabled.
The "Legal Problems" section of crypt.texi is dropped, and the
introductory text is shifted down to the "Encrypting Passwords"
section; the next patch will add some new introductory text.

Also, it is no longer true that crypt.texi's top @node needs to have
no pointers.  That was a vestige of crypt/ being an add-on.  (makeinfo
itself doesn't need @node pointers anymore, but the scripts that
assemble the libc manual's topmost node rely on each chapter-level
node having them.)