* string_append_c(): use another name for buf

* string_append_c(): use another name for buf
* imap_roomname():
  * 256 is double ROOMNAMELEN; calculate it here to show the relation in place
  * extract_token() gives us -1 or the stringlength, use it instead of doing another strlen a line below
  * use safestrncpy()
* do_imap_match():
  * while lowercasing run till \0 rather than doing strlen above
  * respect the size of the target buffer...
  * For-loop: assign pointers here, since we start using them here; use IsEmptyStr all over the place (also some forgotten ones...)