Function: ethio-replace-space

Replace ASCII spaces with Ethiopic word separators in the region.

In the specified region, replace word separators surrounded by two
Ethiopic characters, depending on the first argument CH, which should
be 1, 2, or 3.

If CH = 1, word separator will be replaced with an ASCII space.
If CH = 2, with two ASCII spaces.
If CH = 3, with the Ethiopic colon-like word separator.

The 2nd and 3rd arguments BEGIN and END specify the region. (fn CH BEGIN END)