Reviewing the use of <informalexample> and <screen>.

In some cases <informalexample> wasn't appropriate, and the markup was
changed to <programlisting> or other.

In some cases there were spurious <para> elements before and after the
<informalexample>. These were removed.

Reformatted text within <screen> elements because the whitespace *is*
significant.

Added <prompt> and <userinput> elements within <screen> where necessary.

If I spotted inappropriate use of markup within the immediate vicinity
of the <informalexample> elements then I fixed that (mostly the use of
<emphasis remap="...">).

This is part one of these changes -- there's a load of them, and this
goes up to line 11,284 or thereabouts, roughly one third of the way
through.