Update ZTS to work on FreeBSD

Update the common ZTS scripts and individual test cases as needed 
in order to allow them to be run on FreeBSD.  The high level goal
is to provide compatibility wrappers whenever possible to minimize
changes to individual test cases.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9692

tests/zfs-tests/cmd/mktree/mktree.c

@@ -30,7 +30,9 @@
 #include <unistd.h>
 #include <string.h>
 #include <fcntl.h>
+#ifdef __linux__
 #include <sys/xattr.h>
+#endif
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/param.h>
@@ -176,11 +178,13 @@ crtfile(char *pname)
 		exit(errno);
 	}
 
+#ifdef __linux__
 	if (fsetxattr(fd, "user.xattr", pbuf, 1024, 0) < 0) {
 		(void) fprintf(stderr, "fsetxattr(fd, \"xattr\", pbuf, "
 		    "1024, 0) failed.\n[%d]: %s.\n", errno, strerror(errno));
 		exit(errno);
 	}
+#endif
 
 	(void) close(fd);
 	free(pbuf);