pg_combinebackup --clone doesn't work

The pg_combinebackup --clone option currently doesn't work at all.  Even 
on systems where it should it be supported, you'll always get a "file 
cloning not supported on this platform" error.

The reason is this checking code in pg_combinebackup.c:

#if (defined(HAVE_COPYFILE) && defined(COPYFILE_CLONE_FORCE)) || \
     (defined(__linux__) && defined(FICLONE))

         if (opt.dry_run)
             pg_log_debug("would use cloning to copy files");
         else
             pg_log_debug("will use cloning to copy files");

#else
         pg_fatal("file cloning not supported on this platform");
#endif

The problem is that this file does not include the appropriate OS header 
files that would define COPYFILE_CLONE_FORCE or FICLONE, respectively.

The same problem also exists in copy_file.c.  (That one has the right 
header file for macOS but still not for Linux.)

This should be pretty easy to fix up, and we should think about some 
ways to refactor this to avoid repeating all these OS-specific things a 
few times.  (The code was copied from pg_upgrade originally.)

But in the short term, how about some test coverage?  You can exercise 
the different pg_combinebackup copy modes like this:

diff --git a/src/test/perl/PostgreSQL/Test/Cluster.pm 
b/src/test/perl/PostgreSQL/Test/Cluster.pm
index 83f385a4870..7e8dd024c82 100644
--- a/src/test/perl/PostgreSQL/Test/Cluster.pm
+++ b/src/test/perl/PostgreSQL/Test/Cluster.pm
@@ -848,7 +848,7 @@ sub init_from_backup
         }

         local %ENV = $self->_get_env();
-       my @combineargs = ('pg_combinebackup', '-d');
+       my @combineargs = ('pg_combinebackup', '-d', '--clone');
         if (exists $params{tablespace_map})
         {
             while (my ($olddir, $newdir) = each %{ 
$params{tablespace_map} })

We could do something like what we have for pg_upgrade, where we can use 
the environment variable PG_TEST_PG_UPGRADE_MODE to test the different 
copy modes.  We could turn this into a global option.  (This might also 
be useful for future work to use file cloning elsewhere, like in CREATE 
DATABASE?)

Also, I think it would be useful for consistency if pg_combinebackup had 
a --copy option to select the default mode, like pg_upgrade does.