pgsql: Fix similar_escape() so that SIMILAR TO works properly for

Log Message:
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Fix similar_escape() so that SIMILAR TO works properly for patterns involving
alternatives ("|" symbol). The original coding allowed the added ^ and $
constraints to be absorbed into the first and last alternatives, producing
a pattern that would match more than it should. Per report from Eric Noriega.

I also changed the pattern to add an ARE director ("***:"), ensuring that
SIMILAR TO patterns do not change behavior if regex_flavor is changed. This
is necessary to make the non-capturing parentheses work, and seems like a
good idea on general principles.

Back-patched as far as 7.4. 7.3 also has the bug, but a fix seems impractical
because that version's regex engine doesn't have non-capturing parens.

Tags:
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REL7_4_STABLE

Modified Files:
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pgsql/src/backend/utils/adt:
regexp.c (r1.49.4.2 -> r1.49.4.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/regexp.c.diff?r1=1.49.4.2&r2=1.49.4.3)