New lua-variable: completion_chars

  This is used to tokenize input for tab-completion.

  This is part of #90, and is documented here:

            http://lumail.org/lua/completion_chars.html
  Tokenize input on ",".

  This updates #90.
  Removed obsolete comment.
  Moved variable bindings into their own unit.
  Compile the new variables.{cc h}
  Removed binding code, now it lives elsewhere.
  Reorder functions to be alphabetic, and grouped.
  Include the missing debug.h header.

  This allows debug-builds to compile.
  Ensure that the sent_mail path is a maildir.
  export ESCDELAY=0 - to allow Esc to cancel completion immediately
  Be explicit about ncursesw include path.
  Remove the logging of found entries.

  squeeze doesn't like my usage of std::to_string( size_t X);