Problem: An accidental 'd' was added to remove, making it "removed".

Fix spelling.

Problem: A test in t0028-content-backing-none.t incorrectly
calls checkpoint_put when it should call checkpoint_get.

Fix invalid test.

Problem:  The typical message unpack style is to place key names
and storage pointers on the same line, but that is not done in
several locations in the content and content backing modules.

Correct code style to be more consistent to the rest of flux-core.

Problem: A backing store is required for content.flush but it
is not required for content.checkpoint-put.  This is inconsistent
and can lead to checkpointing problems done the line.

Require content.checkpoint-put to only work if there is a backing
store available.  As a consequence, remove code that handled
"cached" checkpoints when a backing store is not available.

Fixes flux-framework#6251

Problem: Now that the content backing store is required for checkpoints,
many tests fail.

Remove tests that previously assumed that checkpointing worked without
a content backing store.  Adjust some tests that now have an new
error message.

Problem: There is no coverage to ensure that the "none" backing
store works identically to when no backing store is never loaded.

Add coverage in t0028-content-backing-none.t.

Problem: There is no coverage to ensure FLUX_KVS_SYNC fails when
there is no longer space on disk.

Add coverage to t0090-content-enospc.t.

Problem: There is no coverage to ensure FLUX_KVS_SYNC does not
work if there is no backing store.

Add coverage in t1010-kvs-commit-sync.t.

Problem: When the KVS module is unloaded, a checkpoint of the root
reference is attempted.  However, a content.flush is not done
beforehand.  This could result in an invalid checkpoint reference
as data is not guaranteed to be flushed to the backing store.

Solution: Call content.flush before checkpointing.

Fixes flux-framework#6237