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

lib/libzfs/os/freebsd/libzfs_compat.c

@@ -193,8 +193,6 @@ execvpe(const char *name, char * const argv[], char * const envp[])
 	return (execvPe(name, path, argv, envp));
 }
 
-#define	ERRBUFLEN 1024
-
 static __thread char errbuf[ERRBUFLEN];
 
 const char *