Introduce TLSEXT_STATUSTYPE_nothing constant

The existing code used the magic number -1 to represent the absence of
a status_type in the extension. This commit replaces it with a macro.

Perl changes reviewed by Richard Levitte. Non-perl changes reviewed by Rich
Salz

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>