Improvements to the 'compatibility' property

Several improvements to the operation of the 'compatibility' property:

1) Improved handling of unrecognized features:
Change the way unrecognized features in compatibility files are handled.

 * invalid features in files under /usr/share/zfs/compatibility.d
   only get a warning (as these may refer to future features not yet in
   the library),
 * invalid features in files under /etc/zfs/compatibility.d
   get an error (as these are presumed to refer to the current system).

2) Improved error reporting from zpool_load_compat.
Note: slight ABI change to zpool_load_compat for better error reporting.

3) compatibility=legacy inhibits all 'zpool upgrade' operations.

4) Detect when features are enabled outside current compatibility set
   * zpool set compatibility=foo <-- print a warning
   * zpool set feature@xxx=enabled <-- error
   * zpool status <-- indicate this state

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Colm Buckley <colm@tuatha.org>
Closes #11861

include/libzfs.h

@@ -393,6 +393,7 @@ typedef enum {
 	ZPOOL_STATUS_REBUILD_SCRUB,	/* recommend scrubbing the pool */
 	ZPOOL_STATUS_NON_NATIVE_ASHIFT,	/* (e.g. 512e dev with ashift of 9) */
 	ZPOOL_STATUS_COMPATIBILITY_ERR,	/* bad 'compatibility' property */
+	ZPOOL_STATUS_INCOMPATIBLE_FEAT,	/* feature set outside compatibility */
 
 	/*
 	 * Finally, the following indicates a healthy pool.
@@ -922,14 +923,14 @@ extern int zpool_disable_datasets(zpool_handle_t *, boolean_t);
  */
 typedef enum {
 	ZPOOL_COMPATIBILITY_OK,
-	ZPOOL_COMPATIBILITY_READERR,
+	ZPOOL_COMPATIBILITY_WARNTOKEN,
+	ZPOOL_COMPATIBILITY_BADTOKEN,
 	ZPOOL_COMPATIBILITY_BADFILE,
-	ZPOOL_COMPATIBILITY_BADWORD,
 	ZPOOL_COMPATIBILITY_NOFILES
 } zpool_compat_status_t;
 
 extern zpool_compat_status_t zpool_load_compat(const char *,
-    boolean_t *, char *, char *);
+    boolean_t *, char *, size_t);
 
 #ifdef __FreeBSD__