pgsql: Fix string_to_array() to correctly handle the case where there

Log Message:
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Fix string_to_array() to correctly handle the case where there are
overlapping possible matches for the separator string, such as
string_to_array('123xx456xxx789', 'xx').
Also, revise the logic of replace(), split_part(), and string_to_array()
to avoid O(N^2) work from redundant searches and conversions to pg_wchar
format when there are N matches to the separator string.
Backpatched the full patch as far as 8.0. 7.4 also has the bug, but the
code has diverged a lot, so I just went for a quick-and-dirty fix of the
bug itself in that branch.

Modified Files:
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pgsql/src/backend/utils/adt:
varlena.c (r1.151 -> r1.152)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c.diff?r1=1.151&r2=1.152)