Fix Samba share access

[lantis1008]

• Revert removal of "valid users" option
  This was not tested thoroughly enough by me. Causes any valid user credentials to gain access to ALL shares, despite what access restrictions are in place
• Change samba guest account to "nobody" from "root"
  On Gargoyle it probably does not matter, but this is more conventional. If nobody account was compromised it has no valid login anyway
• Add guest to valid users when guest access is selected
  This seems to be required to work properly when the "valid users" option is reinstated.