Remove configure probes for symlink/readlink, and dead code.

symlink() and readlink() are in SUSv2 and all targeted Unix systems have
them.  We have partial emulation on Windows.  Code that raised runtime
errors on systems without it has been dead for years, so we can remove
that and also references to such systems in the documentation.

Define HAVE_READLINK and HAVE_SYMLINK macros on Unix.  Our Windows
replacement functions based on junction points can't be used for
relative paths or for non-directories, so the macros can be used to
check for full symlink support.  The places that deal with tablespaces
can just use symlink functions without checking the macros.  (If they
did check the macros, they'd need to provide an #else branch with a
runtime or compile time error, and it'd be dead code.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

src/common/exec.c

@@ -250,6 +250,10 @@ find_my_exec(const char *argv0, char *retpath)
  * Note: we are not particularly tense about producing nice error messages
  * because we are not really expecting error here; we just determined that
  * the symlink does point to a valid executable.
+ *
+ * Here we test HAVE_READLINK, which excludes Windows.  There's no point in
+ * using our junction point-based replacement code for this, because that only
+ * works for directories.
  */
 static int
 resolve_symlinks(char *path)