Allow the use of completion-tables.

Allow the use of completion-tables.
(pcomplete-std-complete): New command.
(pcomplete-dirs-or-entries): Use a single call to pcomplete-entries.
(pcomplete--here): Use a function for `form' rather than an expression,
so it can be byte-compiled.
(pcomplete-here, pcomplete-here*): Adjust accordingly.
Add edebug declaration.
(pcomplete-show-completions): Remove unused var `curbuf'.
(pcomplete-do-complete, pcomplete-stub):
Don't assume `completions' is a list of	strings any more.