Rewrite of the Libshare/SMBFS
* Add support for options to 'sharesmb'.
  Fixes: #1182
  + Move nfs.c:foreach_nfs_shareopt() to libshare.c:foreach_shareopt()
    so that it can be (re)used in smb.c (and later iscsi.c).
  + Call net(8) with the guest_ok option.
  + Rewrite the sharesmb part of the zfs(8) manpage.
    + Add more examples
    + Inform about the (new) options to sharesmb.
  + Add STDERR_VERBOSE to libzfs_run_process() so that we
    can catch any errors from net(8).
  + Don't call smb_retrieve_shares() after enabling a share.
    No point, it's done when really needed anyway.
  + Support 'y', 'yes' and 'true' for guest_ok.
  * If sharesmb=on, just exit cleanly directly at the top of
    get_smb_shareopts_cb().
  * Remove param 2 and 3 from smb_enable_share_one(). That is
    availible in the first param - the impl_share struct.
  * Extra debugging added in smb_enable_share_one() - print
    all our options just before we call net(8).
* Add support for optional, extra script /sbin/zfs_share_smbfs
  to run after shareing is done.
  + Add an example zfs_share_smbfs script
* 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>'.
  + Add an example on how to modify a share after ZFS share and what man
    pages to use for more info.
* 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'.
* Run cstyle.pl to fix any code style problems.
* Output error if 'net' can't talk to samba.
* Improve error message if 'net' isn't executable.
* Move the checks from smb_available() to libshare_smb_init()
  to avoid doing them multiple times.
* Convert homemade linked list to the ZFS versions list_* functions (which uses a link_t).
* Script to list SMB shares added.
* Comment out a call to zfs_unshare_smb(). Don't seem to be needed.
  This because zfs_unshare_smb() is called twice if setting 'sharesmb=off' - once
  'somewhere else' and once here (this one AFTER the share have been unshared). That
  makes this one fail! Don't know if this is correct, but it works to comment out this...