Update read-only status of newline before point.
The `fence' read-only property is used to indicate that a newline
is read-only for no other reason than to "fence off" a
following front-sticky read-only region. This is used to
implement comint read-only prompts. If the text after a newline
changes, the read-only status of that newline may need updating.
That is what this function does.
This function does nothing if point is not at the beginning of a
line, or is at the beginning of the accessible portion of the buffer.
Otherwise, if the character after point has a front-sticky
read-only property, then the preceding newline is given a
read-only property of `fence', unless it already is read-only.
If the character after point does not have a front-sticky
read-only property, any read-only property of `fence' on the preceding newline is removed. (fn)