Add libtpool (thread pools)

OpenZFS provides a library called tpool which implements thread
pools for user space applications.  Porting this library means
the zpool utility no longer needs to borrow the kernel mutex and
taskq interfaces from libzpool.  This code was updated to use
the tpool library which behaves in a very similar fashion.

Porting libtpool was relatively straight forward and minimal
modifications were needed.  The core changes were:

* Fully convert the library to use pthreads.
* Updated signal handling.
* lmalloc/lfree converted to calloc/free
* Implemented portable pthread_attr_clone() function.

Finally, update the build system such that libzpool.so is no
longer linked in to zfs(8), zpool(8), etc.  All that is required
is libzfs to which the zcommon soures were added (which is the way
it always should have been).  Removing the libzpool dependency
resulted in several build issues which needed to be resolved.

* Moved zfeature support to module/zcommon/zfeature_common.c
* Moved ratelimiting to to module/zfs/zfs_ratelimit.c
* Moved get_system_hostid() to lib/libspl/gethostid.c
* Removed use of cmn_err() in zcommon source
* Removed dprintf_setup() call from zpool_main.c and zfs_main.c
* Removed highbit() and lowbit()
* Removed unnecessary library dependencies from Makefiles
* Removed fletcher-4 kstat in user space
* Added sha2 support explicitly to libzfs
* Added highbit64() and lowbit64() to zpool_util.c

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6442

module/zcommon/zfs_comutil.c

@@ -207,85 +207,10 @@ const char *zfs_history_event_names[ZFS_NUM_LEGACY_HISTORY_EVENTS] = {
 	"pool split",
 };
 
-/*
- * Initialize rate limit struct
- *
- * rl:		zfs_ratelimit_t struct
- * burst:	Number to allow in an interval before rate limiting
- * interval:	Interval time in seconds
- */
-void
-zfs_ratelimit_init(zfs_ratelimit_t *rl, unsigned int burst,
-    unsigned int interval)
-{
-	rl->count = 0;
-	rl->start = 0;
-	rl->interval = interval;
-	rl->burst = burst;
-	mutex_init(&rl->lock, NULL, MUTEX_DEFAULT, NULL);
-}
-
-/*
- * Finalize rate limit struct
- *
- * rl:		zfs_ratelimit_t struct
- */
-void
-zfs_ratelimit_fini(zfs_ratelimit_t *rl)
-{
-	mutex_destroy(&rl->lock);
-}
-
-/*
- * Re-implementation of the kernel's __ratelimit() function
- *
- * We had to write our own rate limiter because the kernel's __ratelimit()
- * function annoyingly prints out how many times it rate limited to the kernel
- * logs (and there's no way to turn it off):
- *
- * 	__ratelimit: 59 callbacks suppressed
- *
- * If the kernel ever allows us to disable these prints, we should go back to
- * using __ratelimit() instead.
- *
- * Return values are the same as __ratelimit():
- *
- * 0: If we're rate limiting
- * 1: If we're not rate limiting.
- */
-int
-zfs_ratelimit(zfs_ratelimit_t *rl)
-{
-	hrtime_t now;
-	hrtime_t elapsed;
-	int rc = 1;
-
-	mutex_enter(&rl->lock);
-
-	now = gethrtime();
-	elapsed = now - rl->start;
-
-	rl->count++;
-	if (NSEC2SEC(elapsed) >= rl->interval) {
-		rl->start = now;
-		rl->count = 0;
-	} else {
-		if (rl->count >= rl->burst) {
-			rc = 0;	/* We're ratelimiting */
-		}
-	}
-	mutex_exit(&rl->lock);
-
-	return (rc);
-}
-
 #if defined(_KERNEL) && defined(HAVE_SPL)
 EXPORT_SYMBOL(zfs_allocatable_devs);
 EXPORT_SYMBOL(zpool_get_rewind_policy);
 EXPORT_SYMBOL(zfs_zpl_version_map);
 EXPORT_SYMBOL(zfs_spa_version_map);
 EXPORT_SYMBOL(zfs_history_event_names);
-EXPORT_SYMBOL(zfs_ratelimit_init);
-EXPORT_SYMBOL(zfs_ratelimit_fini);
-EXPORT_SYMBOL(zfs_ratelimit);
 #endif