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

src/include/port.h

@@ -392,6 +392,23 @@ extern double rint(double x);
 extern int	inet_aton(const char *cp, struct in_addr *addr);
 #endif
 
+/*
+ * Windows and older Unix don't have pread(2) and pwrite(2).  We have
+ * replacement functions, but they have slightly different semantics so we'll
+ * use a name with a pg_ prefix to avoid confusion.
+ */
+#ifdef HAVE_PREAD
+#define pg_pread pread
+#else
+extern ssize_t pg_pread(int fd, void *buf, size_t nbyte, off_t offset);
+#endif
+
+#ifdef HAVE_PWRITE
+#define pg_pwrite pwrite
+#else
+extern ssize_t pg_pwrite(int fd, const void *buf, size_t nbyte, off_t offset);
+#endif
+
 #if !HAVE_DECL_STRLCAT
 extern size_t strlcat(char *dst, const char *src, size_t siz);
 #endif