pgsql: Implement regexp_match(), a simplified alternative to regexp_mat

Implement regexp_match(), a simplified alternative to regexp_matches().

regexp_match() is like regexp_matches(), but it disallows the 'g' flag
and in consequence does not need to return a set.  Instead, it returns
a simple text array value, or NULL if there's no match.  Previously people
usually got that behavior with a sub-select, but this way is considerably
more efficient.

Documentation adjusted so that regexp_match() is presented first and then
regexp_matches() is introduced as a more complicated version.  This is
a bit historically revisionist but seems pedagogically better.

Still TODO: extend contrib/citext to support this function.

Emre Hasegeli, reviewed by David Johnston

Discussion: <CAE2gYzy42sna2ME_e3y1KLQ-4UBrB-eVF0SWn8QG39sQSeVhEw@mail.gmail.com>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/cf9b0fea5f6d1bcc9b2c66f5c30ecb04684a0919

Modified Files
--------------
doc/src/sgml/func.sgml                     | 155 ++++++++++++++++++++---------
src/backend/catalog/information_schema.sql |   2 +-
src/backend/utils/adt/regexp.c             | 137 +++++++++++++++++--------
src/include/catalog/catversion.h           |   2 +-
src/include/catalog/pg_proc.h              |   8 +-
src/include/utils/builtins.h               |   2 +
src/test/regress/expected/regex.out        |  28 ++++++
src/test/regress/expected/strings.out      |   4 +-
src/test/regress/sql/regex.sql             |   7 ++
9 files changed, 252 insertions(+), 93 deletions(-)