exec

Using the new load/gettext.js:
A sysop can override these additional strings (that contain Ctrl-A sequences)
now by assigning to the following keys:
- shell_prompt_begin
- shell_prompt_middle
- shell_main_prompt_end
- shell_file_prompt_end

Also, a sysop can now put these keys in the [default.js] section of the file
(rather than [js]) if desired, to avoid conflict with other scripts/shells.

Added for vela025 and their BBC Micro Acron mode 7 language support mod.
There are other strings still hard-coded in default.js and notably, in
load/shell_lib.js that are not (yet) customizable. Let's try this change first.