Add support for optional, extra script /sbin/zfs_share_smbfs
to run after shareing is done.
+ Add an example zfs_share_smbfs script
Make sure 'everyone' knows about changes to the sharetab.
+ Instead of opening a temporary sharetab and then renaming
  it, just open it with freopen() writeable and write the info.
  + If sharetab isn't opened, open it with fopen() instead of
    freopen() to avoid segfault if it doesn't exist.
Don't open sharetab in parse_sharetab() - it's already open!
+ Reuse the handle, fseek to beginning and read...
The option sharesmb should not be inherited by childs.
Samba is perfectly able to show child file systems...
Use dataset name in comment, not the smb share name.
Use REGISTRY SHARES instead of USER SHARES
+ These are a lot more configurable (more possibility to customize the share).
+ Remove the 'acl' option and replace it with 'writeable' instead.
+ On some machines, there is a "global" share. This isn't really a share,
  but the possibility to modify global options. So first get a line of
  _real_ shares using 'net conf listshares' and _then_ cyckle through
  them retrieving info with 'net conf showshare <share>'.
Speedups (less share retreivals) and make sure the 'net' command is executable.
* Since smb_shares is global, we only retreive shares if it's NULL
  in smb_retrieve_shares(). It should limit the number of times we
  fork() and read shares (which takes time).
* If the net command don't exist (is executable), don't even register
  the smb fstype in libshare_smb_init().
* access() fix - check if NET_CMD_PATH is _executable_ not just 'exists'.