pgsql: Move our p{read,write}v replacements into their own files.

Move our p{read,write}v replacements into their own files.

macOS's ranlib issued a warning about an empty pread.o file with the
previous arrangement, on systems new enough to require no replacement
functions.  Let's go back to using configure's AC_REPLACE_FUNCS system
to build and include each .o in the library only if it's needed, which
requires moving the *v() functions to their own files.

Also move the _with_retry() wrapper to a more permanent home.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1283127.1610554395%40sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0d56acfbaa799553c0c6ea350fd6e68d81025994

Modified Files
--------------
configure                     |  54 ++++++++++++++++++++-
configure.ac                  |   8 ++--
src/backend/storage/file/fd.c |  65 +++++++++++++++++++++++++
src/include/storage/fd.h      |   5 ++
src/port/Makefile             |   2 -
src/port/pread.c              |  43 +----------------
src/port/preadv.c             |  58 +++++++++++++++++++++++
src/port/pwrite.c             | 107 +-----------------------------------------
src/port/pwritev.c            |  58 +++++++++++++++++++++++
src/tools/msvc/Mkvcbuild.pm   |   2 +-
10 files changed, 248 insertions(+), 154 deletions(-)