libzfs: return (allocated) strings instead of filling buffers

This also expands the zfs version output from 127 characters to However
Many Are Actually Set

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13330