Started to dig into Emacs' completion engine.  Separated code that get the boundaries of the text to be completed from the code which actually gets the completion candidates.

Furthermore "wordbreak completion" is turned off for now.
Reworked `bashcomp-process-tokens', renamed `bashcomp-parse-current-command' to `bashcomp-extract-current-command'.
Uncluttered `bashcomp-completion-at-point'.
Hurray, completion is "lazy" now and resorts to Bash once per completion only.
Use macro `lazy-completion-table'.
Renamed `bashcomp-optimize-stub' to `bashcomp-generalize-stub'.
Deactivated "wordbreak" completion for now.