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

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 <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #7285