For Perl compiled with threads, without PERL_SET_INTERP() the PL_curinterp
remains set to the first interpreter created (that is, one created at
original start).  As a result after a reload Perl thinks that operations
are done withing a thread, and, most notably, denies to change environment.

For example, the following code properly works on original start,
but fails after a reload:

    perl 'sub {
        my $r = shift;

        $r->send_http_header("text/plain");

        $ENV{TZ} = "UTC";
        $r->print("tz: " . $ENV{TZ} . " (localtime " . (localtime()) . ")\n");
        $ENV{TZ} = "Europe/Moscow";
        $r->print("tz: " . $ENV{TZ} . " (localtime " . (localtime()) . ")\n");

        return OK;
    }';

To fix this, PERL_SET_INTERP() added anywhere where PERL_SET_CONTEXT()
was previously used.

Note that PERL_SET_INTERP() doesn't seem to be documented anywhere.
Yet it is used in some other software, and also seems to be the only
solution possible.

As the pointer to the first argument was tested instead of the argument
itself, array of arguments was always created, even if there were no
arguments.  Fix is to test args[0] instead of args.

Found by Coverity (CID 1356862).

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.

When prototyping behavior is not explicitly specified, xsubpp emits
a message to stderr asking to do so (see ticket #608).

perl_parse() function expects argv/argc-style argument list,
which according to the C standard must be NULL-terminated,
that is: argv[argc] == NULL.

This change fixes a crash (SIGSEGV) that could happen because
of the buffer overrun during perl module initialization.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

Casts between pointers and integers produce warnings on size mismatch.  To
silence them, cast to (u)intptr_t should be used.  Prevoiusly, casts to
ngx_(u)int_t were used in some cases, and several ngx_int_t expressions had
no casts.

As of now it's mostly style as ngx_int_t is defined as intptr_t.

While ngx_get_full_name() might have a bit more descriptive arguments,
the ngx_conf_full_name() is generally easier to use when parsing
configuration and limits exposure of cycle->prefix / cycle->conf_prefix
details.

The ngx_get_full_name() function takes more readable arguments list.

As per perlxs, C preprocessor directives should be at the first
non-whitespace of a line to avoid interpreting them as comments.

#if and #endif are moved so that there are no blank lines before them
to retain them as part of the function body.