msg_filter is now a function.

  (not used).
  msg_filter works.
  Move get_key_name to CScreen.

  This allows us to use KEY_LEFT + KEY_RIGHT when handling
 ambiguities in TAB-completion.
  Buffer writes to our logfile, to speedup.

  Running lumail-debug under gprof the logfile writing was a
 bottleneck.  Probably because we log too much data.  Buffering
 writes in batches of 50+ lines speeds things up.
  Assert /bin/cat exists, before using it.

  Though this is unlikely to fail we should consider it.
  Changed the sorting code for CMaildirs.

  After profiling getting the paths and running transform(tolower)
 on them is slower than just using strcasecmp - and the latter has
 the same result.

  This is an optimization.
  TODO: Remove the padding on flags.

  has_flag() is an optimization blackspot.  It needs to be updated
 and the padding loop is probably a contributor to the slowness.
  Cache the name of a maildir.

  Since they are never renamed we can do this safely.
  Speedup: Don't write all debug strings immediately.

  Buffer them and write them in batches, unless forced.
  Force log_message() to immediately flush pending
 debug messages.
  Preparing for a new release.

  Since we've got known-segfaults in the prior release I think
 pushing this out, with the many changes, is a good plan.

  While the RFC2047 stuff is still outstanding that won't be
 a quick fix.
  Removed CMessage::to(), ::from(), ::subject().

  These are mere helpers around ::header().

  This is part of the potential simplification and migration
 to a new MIME-library
  Mark areas needing MIME-updates.