Remove unused view parameter of expand_macros()

It uses curr_view and other_view global variables.
Adjust edit_selection() function

Remove unused view parameter.
Use format_str() function to format string.
Rename edit_selection to format_edit_selection_cmd

Also add function comment.
Add edit_selection() function

This reduces code duplication a bit.
Use format_str() function in grep_menu.c
Add function to get command targets

This reduces code duplication a bit.
Rename files=>targets for get_cmd_target() results
Implement expand() function
Update docs for non-ascii chars in map lhs

Forgot to do that in appropriate branch, so doing it here.
Document expand() builtin function

Thanks to Sebastian Cyprych.
Merge branch 'add-expand-builtin-function'