Change functions which return literals to return `const char*`

get_format_prompt_string() and zpool_state_to_name() return
a string literal which is read-only, thus they should return
`const char*`.

zpool_get_prop_string() returns a non-const string after
successful nv-lookup, and returns a string literal otherwise.
Since this function is designed to be used for read-only purpose,
the return type should also be `const char*`.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tomohiro Kusumi <[email protected]>
Closes #7285 

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