ascii: Print syntax in output single-spaced.

When SET PRINTBACK=ON was in effect, the ASCII output driver would
put a blank line between successive lines of syntax, because each line
was output separately.  This commit fixes that, by causing the output
core to combine successive syntax output items into a single item that
contains multiple lines of text.  This was essentially what the HTML
output driver was doing anyhow, so putting this into the core also
allows removing the corresponding logic from the HTML driver.