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'.