Replace WGETCH() with input_char(), which works the same except it supports an
input buffer. Calling input_macro(s) sets the input buffer to 's'. The input
buffer is returned by input_char() as if it had been typed by the user.
If an error occurs (cmderr()), the input buffer is cleared.
For backward-compatibility, this is actually a non-billable flag, so all
existing entries will be considered billable. The flag is 'n', the UI
flag is 'B', the toggle function is "billable" and the default key binding is
'b'.
For now, this isn't useful for anything.
