libzfs: Define the defecto standard errbuf size

Every errbuf array in libzfs is 1024 chars.

Define ERRBUFLEN in a shared header, and use it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #13539