Implemented zpool sync command

This addition will enable us to sync an open TXG to the main pool
on demand. The functionality is similar to 'sync(2)' but 'zpool sync'
will return when data has hit the main storage instead of potentially
just the ZIL as is the case with the 'sync(2)' cmd.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #6122

include/libzfs_core.h

@@ -21,6 +21,7 @@
 
 /*
  * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2017 Datto Inc.
  */
 
 #ifndef	_LIBZFS_CORE_H
@@ -89,6 +90,8 @@ boolean_t lzc_exists(const char *);
 
 int lzc_rollback(const char *, char *, int);
 
+int lzc_sync(const char *, nvlist_t *, nvlist_t **);
+
 #ifdef	__cplusplus
 }
 #endif