oc-calendar: Retrieve properly event description created on SOGo

As it is stored as plain text without triming string.

The correct comparison here is NSNotFound as stated in Apple documentation.
oc-rtf: Use what RTF Spec 1.9.1 for control words

This reverts 7581c0a.
oc-rtf: Fix current position to match where _bytes pointer is

This avoids to have extra unknown bytes at the end of the RTF document
as _len would have been reached but _current_pos had incorrect less value.