Avoid accessing uninitialized bool_vector words

Avoid accessing uninitialized bool_vector words

Although loading uninitialized works from memory and then ignoring
the result works fine on conventional architectures, it
technically has undefined behavior in C, so redo bool_vector
allocation so that the code never does that.  This can improve
performance when allocating large vectors of nil, since calloc can
clear the memory lazily.
* src/alloc.c (make_clear_bool_vector): New function,
a generalization of make_uninit_bool_vector.
(make_uninit_bool_vector): Use it.
(Fmake_bool_vector): If !INIT, rely on make_clear_bool_vector.
* src/alloc.c (Fbool_vector):
* src/fns.c (Freverse): Don’t access uninitialized bool_vector words.

(cherry picked from commit 79b9f05d3a44b25db9fd3eff9cc002324cfa790c)