Fix backslash escaping mechanism.

Fix backslash escaping mechanism.
Ensuring:
- double backslash can't effectively recurse and form part of another escape
  sequence (because backslash is treated both in unistr.c and prepstdreply.c)
- other non-unicode escapes aren't ignored (because they are turned into
  question marks in unistr.c)
- first lines of included files don't 'escape' escaping because they are
  retrieved in begin_new_*() rather than get_processed_text_line().
- files intended to be included transparently aren't escaped, and upcoming
  lines aren't doubly escaped