Use proper em and en dashes

Here is a patch to make more use of – and — in the docs,
instead of the plain ASCII "-".  I think this is nicer to read in some
cases and in other cases makes the appearance consistent with the text
generated by the DocBook stylesheets (see for example mdash use in the
TOC for the SQL commands).

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Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services