Add -Wextra to gcc devteam warnings

clang already has it; let's flip the switch and deal with the fallout.
Exclude -Wunused-parameter, as we have many places where we keep unused
parameters to conform to a uniform vtable-like interface.
Also exclude -Wmissing-field-initializers; it's okay to rely on
the standard-mandated behavior of filling out with 0/NULL.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3239)
(cherry picked from commit 560ad13c)