Don’t hard-loop on cycles in ‘read’ etc.

Don’t hard-loop on cycles in ‘read’ etc.

Problem for ‘read’ reported by Pip Cet in:
https://lists.gnu.org/r/emacs-devel/2019-08/msg00316.html
* src/fns.c (Frequire): Protect against circular current-load-list.
* src/lread.c (Fget_load_suffixes):
Protect against circular load-suffixes or load-file-rep-suffixes.
(Fload): Protect against circular loads-in-progress.
(openp): Protect against circular PATH and SUFFIXES.
(build_load_history): Protect against circular load-history or
current-load-list.
(readevalloop_eager_expand_eval): Protect against circular SUBFORMS.
(read1): Protect against circular data.
* test/src/lread-tests.el (lread-circular-hash): New test.