skate list should not print long strings with newlines

[ilyagr]

Is your feature request related to a problem? Please describe.
I stored a shell script with skate set script < script, and now skate list
prints out the full thing.

Describe the solution you'd like
I would print something like:

script  #!/bin/sh\n#script comment\n  (2000 chars omitted)

Describe alternatives you've considered
It could print (string with newlines omitted) instead of trying to preview.

The (omitted binary data) could also mention the length.

charm fs could gain a charm fs write function that is similar to skate set.

(I suppose this FR also applies to charm kv set)