Similarly to the change in 5491:74bfa803a5aa (1.5.9), we should accept
properly escaped URIs and unescape them as needed, else it is not possible
to handle URIs with question marks.

Returning NGX_HTTP_INTERNAL_SERVER_ERROR if a perl code died after
sending header will lead to a "header already sent" alert.  To avoid
it, we now check if header was already sent, and return NGX_ERROR
instead if it was.

Variable handlers are not expected to send anything to the client, cannot
sleep or read body, and are not expected to modify the request.  Added
appropriate protection to prevent accidental foot shooting.

When an error happens, the ctx->error bit is now set, and croak()
is called to terminate further processing.  The ctx->error bit is
checked in ngx_http_perl_call_handler() to cancel further processing,
and is also checked in various output functions - to make sure these won't
be called if croak() was handled by an eval{} in perl code.

In particular, this ensures that output chain won't be called after
errors, as filters might not expect this to happen.  This fixes some
segmentation faults under low memory conditions.  Also this stops
request processing after filter finalization or request body reading
errors.

For cases where an HTTP error status can be additionally returned (for
example, 416 (Requested Range Not Satisfiable) from the range filter),
the ctx->status field is also added.

This ensures that correct ctx is always available, including after
filter finalization.  In particular, this fixes a segmentation fault
with the following configuration:

    location / {
        image_filter test;

        perl 'sub {
            my $r = shift;
            $r->send_http_header();
            $r->print("foo\n");
            $r->print("bar\n");
        }';
    }

This also seems to be the only way to correctly handle filter finalization
in various complex cases, for example, when embedded perl is used both
in the original handler and in an error page called after filter
finalization.

unsupported threaded environment, but now they complicate code:
*) perl_clone() requires at least duplicating nginx stash;
*) the multiplicity requires to re-evalute all precompiled subroutines
   and nginx stash in new interpreter context.