Use of `execl()` is prohibited on these platforms

Prevent signed integer overflow.

Fix typo

I misunderstood the return value from `snprintf()`, which is NOT number
of characters written in buffer, but the number of character the buffer
has to have to write the complete output.

But when `MRB_WITHOUT_FLOAT` is set, there's no way to sleep for
sub seconds. So mruby specific `usleep` is provided.

 * Method implementation functions made `static`.
 * Function declaration style has been changed.
 * Unnecessary header file `mrb_sleep.h` removed.
 * Used `mrb_get_args()` instead of self parsing.
 * Indentation kept untouched.

Support verbose mode in bintest.

`argv` may be retrieved from an array whose `ptr` is `NULL` when it`s
empty.

This reverts commit c96def7c.
This change was from my misunderstanding.

This change was inspired by [ruby-bugs#7573]

Avoid root fiber initialization for CRuby.

closes #4085.

Reversing gems final

It causes trouble for safe navigation operator.

But `BasicObject#__send__` is still available from the core.

The bytecode format was updated so the header version constants must be
updated as well.

Unlike `mrb_check_hash_type()` that returns `nil` if the argument is
not a `Hash`, `mrb_ensure_hash_type()` raises a `TypeError` exception.