Implemented sharing datasets via SMB using libshare

Add the initial support for the 'smbshare' option using the
existing libshare infrastructure.  Because this implementation
relies on usershares samba version 3.0.23 is required.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #493

lib/libshare/libshare.c

@@ -36,6 +36,7 @@
 #include <libshare.h>
 #include "libshare_impl.h"
 #include "nfs.h"
+#include "smb.h"
 
 static sa_share_impl_t find_share(sa_handle_impl_t handle,
     const char *sharepath);
@@ -103,6 +104,7 @@ __attribute__((constructor)) static void
 libshare_init(void)
 {
 	libshare_nfs_init();
+	libshare_smb_init();
 
 	/*
 	 * This bit causes /etc/dfs/sharetab to be updated before libzfs gets a