update substring pattern matching syntax

At 
<https://wiki.postgresql.org/wiki/PostgreSQL_vs_SQL_Standard#Obsolete_syntax_for_substring.28.29> 
it is described that the substring pattern matching syntax in PostgreSQL 
does not conform to the current standard.  PostgreSQL implements

     SUBSTRING(text FROM pattern FOR escapechar)

whereas the current standard says

     SUBSTRING(text SIMILAR pattern ESCAPE escapechar)

The former was in SQL99, but the latter has been there since SQL:2003.

It's pretty easy to implement the second form also, so here is a patch 
that does that.

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