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

src/port/pwritev.c

@@ -0,0 +1,58 @@
+/*-------------------------------------------------------------------------
+ *
+ * pwritev.c
+ *	  Implementation of pwritev(2) for platforms that lack one.
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/port/pwritev.c
+ *
+ * Note that this implementation changes the current file position, unlike
+ * the POSIX-like function, so we use the name pg_pwritev().
+ *
+ *-------------------------------------------------------------------------
+ */
+
+
+#include "postgres.h"
+
+#ifdef WIN32
+#include <windows.h>
+#else
+#include <unistd.h>
+#endif
+
+#include "port/pg_iovec.h"
+
+ssize_t
+pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+{
+#ifdef HAVE_WRITEV
+	if (iovcnt == 1)
+		return pg_pwrite(fd, iov[0].iov_base, iov[0].iov_len, offset);
+	if (lseek(fd, offset, SEEK_SET) < 0)
+		return -1;
+	return writev(fd, iov, iovcnt);
+#else
+	ssize_t		sum = 0;
+	ssize_t		part;
+
+	for (int i = 0; i < iovcnt; ++i)
+	{
+		part = pg_pwrite(fd, iov[i].iov_base, iov[i].iov_len, offset);
+		if (part < 0)
+		{
+			if (i == 0)
+				return -1;
+			else
+				return sum;
+		}
+		sum += part;
+		offset += part;
+		if (part < iov[i].iov_len)
+			return sum;
+	}
+	return sum;
+#endif
+}