Compile fixes.
Fix spa.c.
Fix zfs_cmd_t initializers:

	OLD: zfs_cmd_t zc = {"\0", 0, 0, 0, 0, 0, 0, 0, "\0", "\0", "\0"};
	NEW: zfs_cmd_t zc = {"\0"};
Fix declaration of zfs_hold().
Fix a couple more "zfs_cmd_t zc = {..." initializers.
Another zfs_cmd_t initializer.
Export functions in dsl_destroy.

Add dsl_destroy and dsl_userhold to libzpool's Makefile.

Add casts to uint64_t printfs in ztest.c