* fall back to __secure_getenv when secure_getenv is not available

* use secure_getenv for instrumented runtimes

* documentation: warn against setting the setuid or setgid bits on custom bytecode executables

A few more wrappers were added (caml_stat_alloc_noexc, caml_stat_resize_noexc,
caml_stat_calloc_noexc) that do not throw an exception in case of errors and
offer a compatible substitute to the corresponding stdlib functions.

Replace a few occurrences of assert and many occurrences of Assert
by CAMLassert.

In asmrun/spacetime_snapshot.c, the line

  Assert(0);  /* unreachable */

in the function caml_spacetime_shape_table has been removed
rather than replaced because the caml_failwith function called
just above has the noreturn attribute which should be enough to
keep compilers happy.

The same holds in the function caml_spacetime_only_works_for_native_code
defined in byterun/spacetime.c

* Do not enter blocking section during snapshot

* Exit cleanly on SIGINT with spacetime

* Fix segfault with spacetime signal handlers