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