Make sure Gateway.NoFetch applies fully

[magik6k]

#5649 implemented Gateway.NoFetch option and applies it to /api by using global offline coreapi option. This means that read-only commands which don't yet use coreapi aren't covered and need to be converted:

From #5649 (comment):

• /get - will be hard to port, there is a lot of code surrounding it which will need to be tamed. - Refactor ipfs get to use CoreAPI #5943
• /dns - needs to get into coreapi (probably under .Name()) (doesn't really need to be covered)
• /ls - Can be rewritten to use coreapi, but that's going to be quite hard (it can be worked around manually for now)
• /dag/get - needs to be ported to coreapi
• /dag/resolve - same as above