Make force-load-doc-strings work again

Make force-load-doc-strings work again

When load-force-doc-strings is true, read (#$ . POS) as the (unibyte)
string referred to.  This feature was lost by mistake in the recent
nonrecursive reader rewrite.

Noticed by Stefan Monnier.

* src/lread.c (get_lazy_string): New function (code mostly recycled
from an old version).
(read0): Detect (#$ . FIXNUM) and retrieve the string if appropriate.
* test/src/lread-resources/lazydoc.el:
* test/src/lread-tests.el (lread-force-load-doc-strings):
New test.