Better support for external processes.

* [ANSI_EXT,SC] Renamed from module TERM_EXT.
* [ANSI_EXT,NEW] Added ansi_format/[1,2], allowing abbreviated ANSI formatting in a list.
* [ANSI_EXT,NEW] Added variants with newline, i.e., ansi_formatnl/[1,2,3,4].
* [RUN_EXT,NEW] Added open/1 that calls the more specific predicates open_X based on the file type.
* [RUN_EXT,NEW] Added list_external_programs/[0,1] which lists the installation status of the programs that are used by some predicate.
* [RUN_EXT,NEW] Use file_type_program/2 to register programs that can handle specific file types.
* [RUN_EXT,SC] All running processed are now killed when Prolog halts.
* [RUN_EXT,SC] External programs now run in a separate thread.