For each slot, the number of total and used entries, as well as
the number of allocation requests and failures, are tracked.

No functional changes.

Removed code that would cause an endless loop, and removed condition
check that is always false.  The first page in the slot list is
guaranteed to satisfy an allocation.

When estimating the number of pages, do not count memory for slots.
In some cases this gives one extra usable memory page.

No functional changes.

On exit environment allocated from a pool is no longer available, leading
to a segmentation fault if, for example, a library tries to use it from
an atexit() handler.

Fix is to allocate environment via ngx_alloc() instead, and explicitly
free it using a pool cleanup handler if it's no longer used (e.g., on
configuration reload).

In Perl 5.8.6 the default was switched to use putenv() when used as
embedded library unless "PL_use_safe_putenv = 0" is explicitly used
in the code.  Therefore, for modern versions of Perl it is no longer
necessary to restore previous environment when calling perl_destruct().