RTFSprms::set: replace two bool arguments with an enum

The primary motivation is that the two bools allow 4 combinations, but
we only handle 3 of them. Also, caller code is much more readable, now
that a non-default overwrite mode is written explicitly.

Change-Id: Iedf7dee1ab2d33de849aef6d7a51138d35048e5a
Testcase for RtfAttributeOutput::SectionLineNumbering()

lcov pointed out this method wasn't ever invoked in any of the tests.

Change-Id: I6cccf4fc1d870938a2f218bedc0b8d195a299cd1