Added message-prolog and message-epilog and

Added message-prolog and message-epilog and
suppress-context-node-name local params to the write.chunk and
write.text.chunk templates. They enable additional info to be
emitted before/after the "Writing..." message, and enable they
context-node name to be suppressed in output. The reason for
these additions is that the write.text.chunk template is
"repurposed" in manpages output for writing the names of soelim
stub files as well as "real" man pages, and also is used for
writing the manifest file, and it is useful to emit details about
the type of file it is writing each time.