pgsql: Use C99 restrict via pg_restrict, rather than restrict directly.

Use C99 restrict via pg_restrict, rather than restrict directly.

Unfortunately using 'restrict' plainly causes problems with MSVC,
which supports restrict only as '__restrict'. Defining 'restrict' to
'__restrict' unfortunately causes a conflict with MSVC's usage of
__declspec(restrict) in headers.

Therefore define pg_restrict to the appropriate keyword instead, and
replace existing usages.

This replaces the temporary workaround introduced in 36b4b91ba078.

Author: Andres Freund
Discussion: https://postgr.es/m/2656.1507830907@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/91d5f1a4a3e8aea2a6488243bac55806160408fb

Modified Files
--------------
configure | 107 ++++++++++++++++++++++++------------------
configure.in | 15 +++++-
src/include/libpq/pqformat.h | 24 +++++-----
src/include/pg_config.h.in | 4 ++
src/include/pg_config.h.win32 | 22 ++++-----
5 files changed, 101 insertions(+), 71 deletions(-)