api: move _args defintions to api-types.h

api: move _args defintions to api-types.h

This change establishes a new (internal) header, api-types.h, and moves
the existing _args structs from ioctl.h.

This will allow us to share the definitions between the direct-ioctl
and MI interfaces.

The changes are purely a move of the struct definitions, and some
whitespace fixes, and some new file-level comments. None of the structs
are altered.

Fixes https://github.com/linux-nvme/libnvme/issues/373 .

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>