pgsql: Fix SQL:2008 FETCH FIRST syntax to allow parameters.

Fix SQL:2008 FETCH FIRST syntax to allow parameters.

OFFSET <x> ROWS FETCH FIRST <y> ROWS ONLY syntax is supposed to accept
<simple value specification>, which includes parameters as well as
literals. When this syntax was added all those years ago, it was done
inconsistently, with <x> and <y> being different subsets of the
standard syntax.

Rectify that by making <x> and <y> accept the same thing, and allowing
either a (signed) numeric literal or a c_expr there, which allows for
parameters, variables, and parenthesized arbitrary expressions.

Per bug #15200 from Lukas Eder.

Backpatch all the way, since this has been broken from the start.

Discussion: https://postgr.es/m/877enz476l.fsf@news-spur.riddles.org.uk
Discussion: http://postgr.es/m/152647780335.27204.16895288237122418685@wrigleys.postgresql.org

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cf516dc9d69052360f1de390636c220797120d28

Modified Files
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doc/src/sgml/ref/select.sgml | 14 +++++++------
src/backend/parser/gram.y | 49 +++++++++++++++++++++++++++++---------------
2 files changed, 41 insertions(+), 22 deletions(-)