Introduced 2 new poll concepts:

- Closures (polls can be closed for new voting by the pollster)
- Results can have configurable visibility:
  a. Only to voters (and the pollster) - the default
  b. Everyone
  c. Everyone once the poll has closed
  d. Only the pollster

Changes to smb_getmsgtxt():
Main change: poll questions can now be quoted when replying to a posted poll
(the results cannot be quoted).
Also: there's now automatically a blank line inserted between comment header
fields and poll answers or the msg body text.
Also: upon any malloc failure, the function now returns NULL.
New functions: smb_msg_is_from() and smb_addpollclosure().