By default, "map" creates cacheable variables [1].  With this
parameter it creates a non-cacheable variable.

An original idea was to deduce the cacheability of the "map"
variable by checking the cacheability of variables specified
in source and resulting values, but it turned to be too hard.
For example, a cacheable variable can be overridden with the
"set" directive or with the SSI "set" command.  Also, keeping
"map" variables cacheable by default is good for performance
reasons.  This required adding a new parameter.

[1] Before db699978a33f (1.11.0), the cacheability of the
"map" variable could vary depending on the cacheability of
variables specified in resulting values (ticket #1090).
This is believed to be a bug rather than a feature.

No functional changes.

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.

Atom size is the sum of atom header size and atom data size. The
specification says that the first 4 bytes are set to one when
the atom size is greater than the maximum unsigned 32-bit value.
Which means atom header size should be considered when the
comparison takes place between atom data size and 0xffffffff.

The variable contains a list of curves as supported by the client.
Known curves are listed by their names, unknown ones are shown
in hex, e.g., "0x001d:prime256v1:secp521r1:secp384r1".

Note that OpenSSL uses session data for SSL_get1_curves(), and
it doesn't store full list of curves supported by the client when
serializing a session.  As a result $ssl_curves is only available
for new sessions (and will be empty for reused ones).

The variable is only meaningful when using OpenSSL 1.0.2 and above.
With older versions the variable is empty.

The variable contains list of ciphers as supported by the client.
Known ciphers are listed by their names, unknown ones are shown
in hex, e.g., ""AES128-SHA:AES256-SHA:0x00ff".

The variable is fully supported only when using OpenSSL 1.0.2 and above.
With older version there is an attempt to provide some information
using SSL_get_shared_ciphers().  It only lists known ciphers though.
Moreover, as OpenSSL uses session data for SSL_get_shared_ciphers(),
and it doesn't store relevant data when serializing a session.  As
a result $ssl_ciphers is only available for new sessions (and not
available for reused ones) when using OpenSSL older than 1.0.2.

It is not necessary to initialize ctx->ranges in all request, because
ctx->ranges in subrequest will be reassigned to ctx->ranges of main
request.

It configures a threshold in bytes, above which client range
requests are not cached.  In such a case the client's Range
header is passed directly to a proxied server.

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).