pgsql: Provide pg_pread() and pg_pwrite() for random I/O.

Provide pg_pread() and pg_pwrite() for random I/O.

Forward to POSIX pread() and pwrite(), or emulate them if unavailable.
The emulation is not perfect as the file position is changed, so
we'll put pg_ prefixes on the names to minimize the risk of confusion
in future patches that might inadvertently try to mix pread() and read()
on the same file descriptor.

Author: Thomas Munro
Reviewed-by: Tom Lane, Jesper Pedersen
Discussion: https://postgr.es/m/CAEepm=02rapCpPR3ZGF2vW=SBHSdFYO_bz_f-wwWJonmA3APgw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3fd2a7932ef0708dda57369bb20c0499d905cc82

Modified Files
--------------
configure                     | 26 ++++++++++++++++++++
configure.in                  |  2 ++
src/include/pg_config.h.in    |  6 +++++
src/include/pg_config.h.win32 |  6 +++++
src/include/port.h            | 17 +++++++++++++
src/port/pread.c              | 55 +++++++++++++++++++++++++++++++++++++++++++
src/port/pwrite.c             | 55 +++++++++++++++++++++++++++++++++++++++++++
src/tools/msvc/Mkvcbuild.pm   |  1 +
8 files changed, 168 insertions(+)