This affects T.P.RoffChar, T.P.Writers.Roff,
and the Man and Ms writers.
That is, `\(xy` instead of `\[xy]`. This was the original AT&T troff
form and is the most widely supported. The bracketed form causes
problem for some tools, e.g. `makewhatis` on macOS.
Closes#10716.
See #9120.
We need to use `\c` before a `.UR` or `.MT`, to avoid
an extra space, and also after. To ensure that a space
at the beginning of the following line doesn't get swallowed
up, we escape it with `\`.
John MacFarlane