pgsql: Change array_offset to return subscripts, not offsets

Change array_offset to return subscripts, not offsets

... and rename it and its sibling array_offsets to array_position and
array_positions, to account for the changed behavior.

Having the functions return subscripts better matches existing practice,
and is better suited to using the result value as a subscript into the
array directly.  For one-based arrays, the new definition is identical
to what was originally committed.

(We use the term "subscript" in the documentation, which is what we use
whenever we talk about arrays; but the functions themselves are named
using the word "position" to match the standard-defined POSITION()
functions.)

Author: Pavel Stěhule
Behavioral problem noted by Dean Rasheed.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/97690ea6e86c412461dd5dc99953b829564d1a55

Modified Files
--------------
doc/src/sgml/array.sgml                 |   18 +++---
doc/src/sgml/func.sgml                  |   22 +++----
src/backend/utils/adt/array_userfuncs.c |   54 +++++++++-------
src/include/catalog/catversion.h        |    2 +-
src/include/catalog/pg_proc.h           |    6 +-
src/include/utils/array.h               |    6 +-
src/test/regress/expected/arrays.out    |  108 +++++++++++++++++--------------
src/test/regress/sql/arrays.sql         |   39 +++++------
8 files changed, 137 insertions(+), 118 deletions(-)