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().

Dependencies of dynamic modules are added to NGX_ADDON_DEPS (and
it is now used for dynamic modules) to be in line with what happens
in case of static compilation.

To avoid duplication, MAIL_DEPS and STREAM_DEPS are no longer passed
to auto/module when these modules are compiled as dynamic ones.  Mail
and stream dependencies are handled explicitly via corresponding
variables.

In collaboration with Ivan Poluyanov.

IPv6 now compiled-in automatically if support is found.  If there is a need
to disable it for some reason, --with-cc-opt="-DNGX_HAVE_INET6=0" can be used
for this.

Previously flags passed by --with-ld-opt were not used when building perl
module, which meant hardening flags provided by package build systems were not
applied.

The ssl_preread module extracts information from the SSL Client Hello message
without terminating SSL.  Currently, only $ssl_preread_server_name variable
is supported, which contains server name from the SNI extension.