iproto: rename IPROTO_KEY_MAX to iproto_key_MAX

We are going to generate iproto_key enum automatically to generate iproto
constants for Lua as well. This mechanism generates *_MAX constant for enum
using its name, which is in lower case. So let's convert IPROTO_KEY_MAX to
form which is appropriate for enum generator.

Part of #8443

NO_TEST=rename constant
NO_CHANGELOG=internal
NO_DOC=internal